- 11 Jan, 2022 1 commit
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Jeff Layton authored
Now that the fscache API has been reworked and simplified, change ceph over to use it. With the old API, we would only instantiate a cookie when the file was open for reads. Change it to instantiate the cookie when the inode is instantiated and call use/unuse when the file is opened/closed. Also, ensure we resize the cached data on truncates, and invalidate the cache in response to the appropriate events. This will allow us to plumb in write support later. Signed-off-by:
Jeff Layton <jlayton@kernel.org> Signed-off-by:
David Howells <dhowells@redhat.com> Link: https://lore.kernel.org/r/20211129162907.149445-2-jlayton@kernel.org/ # v1 Link: https://lore.kernel.org/r/20211207134451.66296-2-jlayton@kernel.org/ # v2 Link: https://lore.kernel.org/r/163906984277.143852.14697110691303589000.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967188351.1823006.5065634844099079351.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/164021581427.640689.14128682147127509264.stgit@warthog.procyon.org.uk/ # v4
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- 10 Jan, 2022 4 commits
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David Howells authored
Move NFS to using fscache DIO API instead of the old upstream I/O API as that has been removed. This is a stopgap solution as the intention is that at sometime in the future, the cache will move to using larger blocks and won't be able to store individual pages in order to deal with the potential for data corruption due to the backing filesystem being able insert/remove bridging blocks of zeros into its extent list[1]. NFS then reads and writes cache pages synchronously and one page at a time. The preferred change would be to use the netfs lib, but the new I/O API can be used directly. It's just that as the cache now needs to track data for itself, caching blocks may exceed page size... This code is somewhat borrowed from my "fallback I/O" patchset[2]. Changes ======= ver #3: - Restore lost =n fallback for nfs_fscache_release_page()[2]. Signed-off-by:
Dave Wysochanski <dwysocha@redhat.com> Acked-by:
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Dave Wysochanski authored
Change the nfs filesystem to support fscache's indexing rewrite and reenable caching in nfs. The following changes have been made: (1) The fscache_netfs struct is no more, and there's no need to register the filesystem as a whole. (2) The session cookie is now an fscache_volume cookie, allocated with fscache_acquire_volume(). That takes three parameters: a string representing the "volume" in the index, a string naming the cache to use (or NULL) and a u64 that conveys coherency metadata for the volume. For nfs, I've made it render the volume name string as: "nfs,<ver>,<family>,<address>,<port>,<fsidH>,<fsidL>*<,param>[,<uniq>]" (3) The fscache_cookie_def is no more and needed information is passed directly to fscache_acquire_cookie(). The cache no longer calls back into the filesystem, but rather metadata changes are indicated at other times. fscache_acquire_cookie() is passed the same keying and coherency information as before. (4) fscache_enable/disable_cookie() have been removed. Call fscache_use_cookie() and fscache_unuse_cookie() when a file is opened or closed to prevent a cache file from being culled and to keep resources to hand that are needed to do I/O. If a file is opened for writing, we invalidate it with FSCACHE_INVAL_DIO_WRITE in lieu of doing writeback to the cache, thereby making it cease caching until all currently open files are closed. This should give the same behaviour as the uptream code. Making the cache store local modifications isn't straightforward for NFS, so that's left for future patches. (5) fscache_invalidate() now needs to be given uptodate auxiliary data and a file size. It also takes a flag to indicate if this was due to a DIO write. (6) Call nfs_fscache_invalidate() with FSCACHE_INVAL_DIO_WRITE on a file to which a DIO write is made. (7) Call fscache_note_page_release() from nfs_release_page(). (8) Use a killable wait in nfs_vm_page_mkwrite() when waiting for PG_fscache to be cleared. (9) The functions to read and write data to/from the cache are stubbed out pending a conversion to use netfslib. Changes ======= ver #3: - Added missing =n fallback for nfs_fscache_release_file()[1][2]. ver #2: - Use gfpflags_allow_blocking() rather than using flag directly. - fscache_acquire_volume() now returns errors. - Remove NFS_INO_FSCACHE as it's no longer used. - Need to unuse a cookie on file-release, not inode-clear. Signed-off-by: -
David Howells authored
When writing to the server from v9fs_vfs_writepage(), copy the data to the cache object too. To make this possible, the cookie must have its active users count incremented when the page is dirtied and kept incremented until we manage to clean up all the pages. This allows the writeback to take place after the last file struct is released. This is done by taking a use on the cookie in v9fs_set_page_dirty() if we haven't already done so (controlled by the I_PINNING_FSCACHE_WB flag) and dropping the pin in v9fs_write_inode() if __writeback_single_inode() clears all the outstanding dirty pages (conveyed by the unpinned_fscache_wb flag in the writeback_control struct). Inode eviction must also clear the flag after truncating away all the outstanding pages. In the future this will be handled more gracefully by netfslib. Changes ======= ver #3: - Canonicalise the coherency data to make it endianness-independent. ver #2: - Fix an unused-var warning due to CONFIG_9P_FSCACHE=n[1]. Signed-off-by:
Dominique Martinet <asmadeus@codewreck.org> cc: Eric Van Hensbergen <ericvh@gmail.com> cc: Latchesar Ionkov <lucho@ionkov.net> cc: v9fs-developer@lists.sourceforge.net cc: linux-cachefs@redhat.com Link: https://lore.kernel.org/r/163819667027.215744.13815687931204222995.stgit@warthog.procyon.org.uk/ # v1 Link: https://lore.kernel.org/r/163906978015.143852.10646669694345706328.stgit@warthog.procyon.org.uk/ # v2 Link: https://lore.kernel.org/r/163967180760.1823006.5831751873616248910.stgit@warthog.procyon.org.uk/ # v3 Link: https://lore.kernel.org/r/164021574522.640689.13849966660182529125.stgit@warthog.procyon.org.uk/ # v4
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David Howells authored
Change the 9p filesystem to take account of the changes to fscache's indexing rewrite and reenable caching in 9p. The following changes have been made: (1) The fscache_netfs struct is no more, and there's no need to register the filesystem as a whole. (2) The session cookie is now an fscache_volume cookie, allocated with fscache_acquire_volume(). That takes three parameters: a string representing the "volume" in the index, a string naming the cache to use (or NULL) and a u64 that conveys coherency metadata for the volume. For 9p, I've made it render the volume name string as: "9p,<devname>,<cachetag>" where the cachetag is replaced by the aname if it wasn't supplied. This probably needs rethinking a bit as the aname can have slashes in it. It might be better to hash the cachetag and use the hash or I could substitute commas for the slashes or something. (3) The fscache_cookie_def is no more and needed information is passed directly to fscache_acquire_cookie(). The cache no longer calls back into the filesystem, but rather metadata changes are indicated at other times. fscache_acquire_cookie() is passed the same keying and coherency information as before. (4) The functions to set/reset/flush cookies are removed and fscache_use_cookie() and fscache_unuse_cookie() are used instead. fscache_use_cookie() is passed a flag to indicate if the cookie is opened for writing. fscache_unuse_cookie() is passed updates for the metadata if we changed it (ie. if the file was opened for writing). These are called when the file is opened or closed. (5) wait_on_page_bit[_killable]() is replaced with the specific wait functions for the bits waited upon. (6) I've got rid of some of the 9p-specific cache helper functions and called things like fscache_relinquish_cookie() directly as they'll optimise away if v9fs_inode_cookie() returns an unconditional NULL (which will be the case if CONFIG_9P_FSCACHE=n). (7) v9fs_vfs_setattr() is made to call fscache_resize() to change the size of the cache object. Notes: (A) We should call fscache_invalidate() if we detect that the server's copy of a file got changed by a third party, but I don't know where to do that. We don't need to do that when allocating the cookie as we get a check-and-invalidate when we initially bind to the cache object. (B) The copy-to-cache-on-writeback side of things will be handled in separate patch. Changes ======= ver #3: - Canonicalise the cookie key and coherency data to make them endianness-independent. ver #2: - Use gfpflags_allow_blocking() rather than using flag directly. - fscache_acquire_volume() now returns errors. Signed-off-by:
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- 07 Jan, 2022 35 commits
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David Howells authored
Don't send a truncation RPC to the server if we're only shortening data that's in the pagecache and is beyond the server's EOF. Also don't automatically force writeback on setattr, but do wait to store RPCs that are in the region to be removed on a shortening truncation. Signed-off-by:
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David Howells authored
When writing to the server from afs_writepage() or afs_writepages(), copy the data to the cache object too. To make this possible, the cookie must have its active users count incremented when the page is dirtied and kept incremented until we manage to clean up all the pages. This allows the writeback to take place after the last file struct is released. Signed-off-by:
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David Howells authored
Change the afs filesystem to support the new afs driver. The following changes have been made: (1) The fscache_netfs struct is no more, and there's no need to register the filesystem as a whole. There's also no longer a cell cookie. (2) The volume cookie is now an fscache_volume cookie, allocated with fscache_acquire_volume(). This function takes three parameters: a string representing the "volume" in the index, a string naming the cache to use (or NULL) and a u64 that conveys coherency metadata for the volume. For afs, I've made it render the volume name string as: "afs,<cell>,<volume_id>" and the coherency data is currently 0. (3) The fscache_cookie_def is no more and needed information is passed directly to fscache_acquire_cookie(). The cache no longer calls back into the filesystem, but rather metadata changes are indicated at other times. fscache_acquire_cookie() is passed the same keying and coherency information as before, except that these are now stored in big endian form instead of cpu endian. This makes the cache more copyable. (4) fscache_use_cookie() and fscache_unuse_cookie() are called when a file is opened or closed to prevent a cache file from being culled and to keep resources to hand that are needed to do I/O. fscache_use_cookie() is given an indication if the cache is likely to be modified locally (e.g. the file is open for writing). fscache_unuse_cookie() is given a coherency update if we had the file open for writing and will update that. (5) fscache_invalidate() is now given uptodate auxiliary data and a file size. It can also take a flag to indicate if this was due to a DIO write. This is wrapped into afs_fscache_invalidate() now for convenience. (6) fscache_resize() now gets called from the finalisation of afs_setattr(), and afs_setattr() does use/unuse of the cookie around the call to support this. (7) fscache_note_page_release() is called from afs_release_page(). (8) Use a killable wait in nfs_vm_page_mkwrite() when waiting for PG_fscache to be cleared. Render the parts of the cookie key for an afs inode cookie as big endian. Changes ======= ver #2: - Use gfpflags_allow_blocking() rather than using flag directly. - fscache_acquire_volume() now returns errors. Signed-off-by: -
David Howells authored
Add a stat counter of culling events whereby the cache backend culls a file to make space (when asked by cachefilesd in this case) and display in /proc/fs/fscache/stats. Signed-off-by:
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David Howells authored
Add stat counters of no-space events that caused caching not to happen and display in /proc/fs/fscache/stats. Signed-off-by:
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David Howells authored
Remove the block that allowed cachefiles to be compiled but prevented it from actually starting a cache. Signed-off-by:
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David Howells authored
Store the volume coherency data in an xattr and check it when we rebind the volume. If it doesn't match the cache volume is moved to the graveyard and rebuilt anew. Changes ======= ver #4: - Remove a couple of debugging prints. Signed-off-by:
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David Howells authored
Implement the I/O routines for cachefiles. There are two sets of routines here: preparation and actual I/O. Preparation for read involves looking to see whether there is data present, and how much. Netfslib tells us what it wants us to do and we have the option of adjusting shrinking and telling it whether to read from the cache, download from the server or simply clear a region. Preparation for write involves checking for space and defending against possibly running short of space, if necessary punching out a hole in the file so that we don't leave old data in the cache if we update the coherency information. Then there's a read routine and a write routine. They wait for the cookie state to move to something appropriate and then start a potentially asynchronous direct I/O operation upon it. Changes ======= ver #2: - Fix a misassigned variable[1]. Signed-off-by:
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David Howells authored
Implement resizing an object, using truncate and/or fallocate to adjust the object. Signed-off-by:
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David Howells authored
Implement the methods for beginning and ending an I/O operation. When called to begin an I/O operation, we are guaranteed that the cookie has reached a certain stage (we're called by fscache after it has done a suitable wait). If a file is available, we paste a ref over into the cache resources for the I/O routines to use. This means that the object can be invalidated whilst the I/O is ongoing without the need to synchronise as the file pointer in the object is replaced, but the file pointer in the cache resources is unaffected. Ending the operation just requires ditching any refs we have and dropping the access guarantee that fscache got for us on the cookie. Signed-off-by:
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David Howells authored
Implement the wrangling of backing files, including the following pieces: (1) Lookup and creation of a file on disk, using a tmpfile if the file isn't yet present. The file is then opened, sized for DIO and the file handle is attached to the cachefiles_object struct. The inode is marked to indicate that it's in use by a kernel service. (2) Invalidation of an object, creating a tmpfile and switching the file pointer in the cachefiles object. (3) Committing a file to disk, including setting the coherency xattr on it and, if necessary, creating a hard link to it. Note that this would be a good place to use Omar Sandoval's vfs_link() with AT_LINK_REPLACE[1] as I may have to unlink an old file before I can link a tmpfile into place. (4) Withdrawal of open objects when a cache is being withdrawn or a cookie is relinquished. This involves committing or discarding the file. Changes ======= ver #2: - Fix logging of wrong error[1]. Signed-off-by: -
David Howells authored
Implement the ability for the userspace daemon to try and cull a file or directory in the cache. Two daemon commands are implemented: (1) The "inuse" command. This queries if a file is in use or whether it can be deleted. It checks the S_KERNEL_FILE flag on the inode referred to by the specified filename. (2) The "cull" command. This asks for a file or directory to be removed, where removal means either unlinking it or moving it to the graveyard directory for userspace to dismantle. Changes ======= ver #2: - Fix logging of wrong error[1]. - Need to unmark an inode we've moved to the graveyard before unlocking. Signed-off-by: -
David Howells authored
Use an inode flag, S_KERNEL_FILE, to mark that a backing file is in use by the kernel to prevent cachefiles or other kernel services from interfering with that file. Using S_SWAPFILE instead isn't really viable as that has other effects in the I/O paths. Signed-off-by:
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David Howells authored
Use an xattr on each backing file in the cache to store some metadata, such as the content type and the coherency data. Five content types are defined: (0) No content stored. (1) The file contains a single monolithic blob and must be all or nothing. This would be used for something like an AFS directory or a symlink. (2) The file is populated with content completely up to a point with nothing beyond that. (3) The file has a map attached and is sparsely populated. This would be stored in one or more additional xattrs. (4) The file is dirty, being in the process of local modification and the contents are not necessarily represented correctly by the metadata. The file should be deleted if this is seen on binding. Signed-off-by: -
David Howells authored
Implement a function to encode a binary cookie key as something that can be used as a filename. Four options are considered: (1) All printable chars with no '/' characters. Prepend a 'D' to indicate the encoding but otherwise use as-is. (2) Appears to be an array of __be32. Encode as 'S' plus a list of hex-encoded 32-bit ints separated by commas. If a number is 0, it is rendered as "" instead of "0". (3) Appears to be an array of __le32. Encoded as (2) but with a 'T' encoding prefix. (4) Encoded as base64 with an 'E' prefix plus a second char indicating how much padding is involved. A non-standard base64 encoding is used because '/' cannot be used in the encoded form. If (1) is not possible, whichever of (2), (3) or (4) produces the shortest string is selected (hex-encoding a number may be less dense than base64 encoding it). Note that the prefix characters have to be selected from the set [DEIJST@] lest cachefilesd remove the files because it recognise the name. Changes ======= ver #2: - Fix a short allocation that didn't allow for a string terminator[1] Signed-off-by: -
David Howells authored
Implement allocate, get, see and put functions for the cachefiles_object struct. The members of the struct we're going to need are also added. Additionally, implement a lifecycle tracepoint. Signed-off-by:
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David Howells authored
Add tracepoints in cachefiles to monitor when it does various VFS operations, such as mkdir. Signed-off-by:
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David Howells authored
Implement support for creating the directory layout for a volume on disk and setting up and withdrawing volume caching. Each volume has a directory named for the volume key under the root of the cache (prefixed with an 'I' to indicate to cachefilesd that it's an index) and then creates a bunch of hash bucket subdirectories under that (named as '@' plus a hex number) in which cookie files will be created. Signed-off-by:
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David Howells authored
Do the following: (1) Fill out cachefiles_daemon_add_cache() so that it sets up the cache directories and registers the cache with cachefiles. (2) Add a function to do the top-level part of cache withdrawal and unregistration. (3) Add a function to sync a cache. Signed-off-by: -
David Howells authored
Implement a function to get/create structural directories in the cache. This is used for setting up a cache and creating volume substructures. The directory in memory are marked with the S_KERNEL_FILE inode flag whilst they're in use to tell rmdir to reject attempts to remove them. Changes ======= ver #3: - Return an indication as to whether the directory was freshly created. Signed-off-by:
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David Howells authored
Use an inode flag, S_KERNEL_FILE, to mark that a backing file is in use by the kernel to prevent cachefiles or other kernel services from interfering with that file. Alter rmdir to reject attempts to remove a directory marked with this flag. This is used by cachefiles to prevent cachefilesd from removing them. Using S_SWAPFILE instead isn't really viable as that has other effects in the I/O paths. Changes ======= ver #3: - Check for the object pointer being NULL in the tracepoints rather than the caller. Signed-off-by:
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David Howells authored
Provide a function to check how much space there is. This also flips the state on the cache and will signal the daemon to inform it of the change and to ask it to do some culling if necessary. We will also need to subtract the amount of data currently being written to the cache (cache->b_writing) from the amount of available space to avoid hitting ENOSPC accidentally. Signed-off-by:
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David Howells authored
Register a misc device with which to talk to the daemon. The misc device holds a cache set up through it around and closing the device kills the cache. cachefilesd communicates with the kernel by passing it single-line text commands. Parse these and use them to parameterise the cache state. This does not implement the command to actually bring a cache online. That's left for later. Signed-off-by:
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David Howells authored
Implement code to derive a new set of creds for the cachefiles to use when making VFS or I/O calls and to change the auditing info since the application interacting with the network filesystem is not accessing the cache directly. Cachefiles uses override_creds() to change the effective creds temporarily. set_security_override_from_ctx() is called to derive the LSM 'label' that the cachefiles driver will act with. set_create_files_as() is called to determine the LSM 'label' that will be applied to files and directories created in the cache. These functions alter the new creds. Also implement a couple of functions to wrap the calls to begin/end cred overriding. Signed-off-by:
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David Howells authored
Add a macro to report a cache I/O error and to tell fscache that the cache is in trouble. Also add a pointer to the fscache cache cookie from the cachefiles_cache struct as we need that to pass to fscache_io_error(). Signed-off-by:
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David Howells authored
Add two trace points to log errors, one for vfs operations like mkdir or create, and one for I/O operations, like read, write or truncate. Also add the beginnings of a struct that is going to represent a data file and place a debugging ID in it for the tracepoints to record. Signed-off-by:
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David Howells authored
Add support for injecting ENOSPC or EIO errors. This needs to be enabled by CONFIG_CACHEFILES_ERROR_INJECTION=y. Once enabled, ENOSPC on things like write and mkdir can be triggered by: echo 1 >/proc/sys/cachefiles/error_injection and EIO can be triggered on most operations by: echo 2 >/proc/sys/cachefiles/error_injection Signed-off-by: -
David Howells authored
Define the cachefiles_cache struct that's going to carry the cache-level parameters and state of a cache. Define the beginning of the cachefiles_object struct that's going to carry the state for a data storage object. For the moment this is just a debugging ID for logging purposes. Signed-off-by:
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David Howells authored
Introduce basic skeleton of the rewritten cachefiles driver including config options so that it can be enabled for compilation. Signed-off-by:
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David Howells authored
Provide a function to change the size of the storage attached to a cookie, to match the size of the file being cached when it's changed by truncate or fallocate: void fscache_resize_cookie(struct fscache_cookie *cookie, loff_t new_size); This acts synchronously and is expected to run under the inode lock of the caller. Signed-off-by:
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David Howells authored
Provide a function to be called from a network filesystem's releasepage method to indicate that a page has been released that might have been a reflection of data upon the server - and now that data must be reloaded from the server or the cache. This is used to end an optimisation for empty files, in particular files that have just been created locally, whereby we know there cannot yet be any data that we would need to read from the server or the cache. Signed-off-by:
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David Howells authored
Cachefiles has a problem in that it needs to keep the backing file for a cookie open whilst there are local modifications pending that need to be written to it. However, we don't want to keep the file open indefinitely, as that causes EMFILE/ENFILE/ENOMEM problems. Reopening the cache file, however, is a problem if this is being done due to writeback triggered by exit(). Some filesystems will oops if we try to open a file in that context because they want to access current->fs or other resources that have already been dismantled. To get around this, I added the following: (1) An inode flag, I_PINNING_FSCACHE_WB, to be set on a network filesystem inode to indicate that we have a usage count on the cookie caching that inode. (2) A flag in struct writeback_control, unpinned_fscache_wb, that is set when __writeback_single_inode() clears the last dirty page from i_pages - at which point it clears I_PINNING_FSCACHE_WB and sets this flag. This has to be done here so that clearing I_PINNING_FSCACHE_WB can be done atomically with the check of PAGECACHE_TAG_DIRTY that clears I_DIRTY_PAGES. (3) A function, fscache_set_page_dirty(), which if it is not set, sets I_PINNING_FSCACHE_WB and calls fscache_use_cookie() to pin the cache resources. (4) A function, fscache_unpin_writeback(), to be called by ->write_inode() to unuse the cookie. (5) A function, fscache_clear_inode_writeback(), to be called when the inode is evicted, before clear_inode() is called. This cleans up any lingering I_PINNING_FSCACHE_WB. The network filesystem can then use these tools to make sure that fscache_write_to_cache() can write locally modified data to the cache as well as to the server. For the future, I'm working on write helpers for netfs lib that should allow this facility to be removed by keeping track of the dirty regions separately - but that's incomplete at the moment and is also going to be affected by folios, one way or another, since it deals with pages Signed-off-by: -
David Howells authored
Provide a higher-level function than fscache_write() to perform a write from an inode's pagecache to the cache, whilst fending off concurrent writes by means of the PG_fscache mark on a page: void fscache_write_to_cache(struct fscache_cookie *cookie, struct address_space *mapping, loff_t start, size_t len, loff_t i_size, netfs_io_terminated_t term_func, void *term_func_priv, bool caching); If caching is false, this function does nothing except call (*term_func)() if given. It assumes that, in such a case, PG_fscache will not have been set on the pages. Otherwise, if caching is true, this function requires the source pages to have had PG_fscache set on them before calling. start and len define the region of the file to be modified and i_size indicates the new file size. The source pages are extracted from the mapping. term_func and term_func_priv work as for fscache_write(). The PG_fscache marks will be cleared at the end of the operation, before term_func is called or the function otherwise returns. There is an additonal helper function to clear the PG_fscache bits from a range of pages: void fscache_clear_page_bits(struct fscache_cookie *cookie, struct address_space *mapping, loff_t start, size_t len, bool caching); If caching is true, the pages to be managed are expected to be located on mapping in the range defined by start and len. If caching is false, it does nothing. Signed-off-by:
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David Howells authored
Provide a pair of functions to perform raw I/O on the cache. The first function allows an arbitrary asynchronous direct-IO read to be made against a cache object, though the read should be aligned and sized appropriately for the backing device: int fscache_read(struct netfs_cache_resources *cres, loff_t start_pos, struct iov_iter *iter, enum netfs_read_from_hole read_hole, netfs_io_terminated_t term_func, void *term_func_priv); The cache resources must have been previously initialised by fscache_begin_read_operation(). A read operation is sent to the backing filesystem, starting at start_pos within the file. The size of the read is specified by the iterator, as is the location of the output buffer. If there is a hole in the data it can be ignored and left to the backing filesystem to deal with (NETFS_READ_HOLE_IGNORE), a hole at the beginning can be skipped over and the buffer padded with zeros (NETFS_READ_HOLE_CLEAR) or -ENODATA can be given (NETFS_READ_HOLE_FAIL). If term_func is not NULL, the operation may be performed asynchronously. Upon completion, successful or otherwise, (*term_func)() will be called and passed term_func_priv, along with an error or the amount of data transferred. If the op is run asynchronously, fscache_read() will return -EIOCBQUEUED. The second function allows an arbitrary asynchronous direct-IO write to be made against a cache object, though the write should be aligned and sized appropriately for the backing device: int fscache_write(struct netfs_cache_resources *cres, loff_t start_pos, struct iov_iter *iter, netfs_io_terminated_t term_func, void *term_func_priv); This works in very similar way to fscache_read(), except that there's no need to deal with holes (they're just overwritten). The caller is responsible for preventing concurrent overlapping writes. Signed-off-by: -
David Howells authored
Pass more information to the cache on how to deal with a hole if it encounters one when trying to read from the cache. Three options are provided: (1) NETFS_READ_HOLE_IGNORE. Read the hole along with the data, assuming it to be a punched-out extent by the backing filesystem. (2) NETFS_READ_HOLE_CLEAR. If there's a hole, erase the requested region of the cache and clear the read buffer. (3) NETFS_READ_HOLE_FAIL. Fail the read if a hole is detected. Signed-off-by:
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