This and related patches which move mount related
code to fs_context.c has the advantage of
shriking the code in fs/cifs/connect.c (which had
the second most lines of code of any of the files
in cifs.ko and was getting harder to read due
to its size) and will also make it easier to
switch over to the new mount API in the future.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Helps to shrink connect.c and make it more readable
by moving mount related code to fs_context.c and
fs_context.h
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>

This patch moves the parsing of security mount options into
fs_context.ch.  There are no changes to any logic.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>

This will make it easier in the future, but also will allow us to
shrink connect.c which is getting too big, and harder to read
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>

To servers which do not support directory leases (e.g. Samba)
it is wasteful to try to open_shroot (ie attempt to cache the
root directory handle).  Skip attempt to open_shroot when
server does not indicate support for directory leases.

Cuts the number of requests on mount from 17 to 15, and
cuts the number of requests on stat of the root directory
from 4 to 3.
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org> # v5.1+

When mounting with modefromsid mount option, it was possible to
get the error on stat of a fifo or char or block device:
        "cannot stat <filename>: Operation not supported"

Special devices can be stored as reparse points by some servers
(e.g. Windows NFS server and when using the SMB3.1.1 POSIX
Extensions) but when the modefromsid mount option is used
the client attempts to get the ACL for the file which requires
opening with OPEN_REPARSE_POINT create option.
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>

Can be helpful in debugging mount and reconnect issues
Signed-off-by: Samuel Cabrero <scabrero@suse.de>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

TCP server info field server->total_read is modified in parallel by
demultiplex thread and decrypt offload worker thread. server->total_read
is used in calculation to discard the remaining data of PDU which is
not read into memory.

Because of parallel modification, server->total_read can get corrupted
and can result in discarding the valid data of next PDU.
Signed-off-by: Rohith Surabattula <rohiths@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
CC: Stable <stable@vger.kernel.org> #5.4+
Signed-off-by: Steve French <stfrench@microsoft.com>

Don't populate const array smb3_create_tag_posix on the stack but
instead make it static. Makes the object code smaller by 50 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
 150184	  47167	      0	 197351	  302e7	fs/cifs/smb2pdu.o

After:
    text	   data	    bss	    dec	    hex	filename
 150070	  47231	      0	 197301	  302b5	fs/cifs/smb2pdu.o

(gcc version 10.2.0)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

We were setting the uid/gid to the default in each dir entry
in the parsing of the POSIX query dir response, rather
than attempting to map the user and group SIDs returned by
the server to well known SIDs (or upcall if not found).

CC: Stable <stable@vger.kernel.org>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

SMB3 crediting is used for flow control, and it can be useful to
trace for problem determination how many credits were acquired
and for which operation.

Here is an example ("trace-cmd record -e *add_credits"):
cifsd-9522    [010] ....  5995.202712: smb3_add_credits:
	server=localhost current_mid=0x12 credits=373 credits_to_add=10
cifsd-9522    [010] ....  5995.204040: smb3_add_credits:
	server=localhost current_mid=0x15 credits=400 credits_to_add=30
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

There are cases where the server can return a cipher type of 0 and
it not be an error. For example server supported no encryption types
(e.g. server completely disabled encryption), or the server and
client didn't support any encryption types in common (e.g. if a
server only supported AES256_CCM). In those cases encryption would
not be supported, but that can be ok if the client did not require
encryption on mount and it should not return an error.

In the case in which mount requested encryption ("seal" on mount)
then checks later on during tree connection will return the proper
rc, but if seal was not requested by client, since server is allowed
to return 0 to indicate no supported cipher, we should not fail mount.
Reported-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

In crypt_message, when smb2_get_enc_key returns error, we need to
return the error back to the caller. If not, we end up processing
the message further, causing a kernel oops due to unwarranted access
of memory.

Call Trace:
smb3_receive_transform+0x120/0x870 [cifs]
cifs_demultiplex_thread+0xb53/0xc20 [cifs]
? cifs_handle_standard+0x190/0x190 [cifs]
kthread+0x116/0x130
? kthread_park+0x80/0x80
ret_from_fork+0x1f/0x30
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
CC: Stable <stable@vger.kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

update smb encryption code to set 32 byte key length and to
set gcm256 when requested on mount.
Signed-off-by: Steve French <stfrench@microsoft.com>

Now that 256 bit encryption can be negotiated, update
names of the nonces to match the updated official protocol
documentation (e.g. AES_GCM_NONCE instead of AES_128GCM_NONCE)
since they apply to both 128 bit and 256 bit encryption.
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

If server does not support AES-256-GCM and it was required on mount, print
warning message. Also log and return a different error message (EOPNOTSUPP)
when encryption mechanism is not supported vs the case when an unknown
unrequested encryption mechanism could be returned (EINVAL).
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

Add new module load parameter enable_gcm_256. If set, then add
AES-256-GCM (strongest encryption type) to the list of encryption
types requested. Put it in the list as the second choice (since
AES-128-GCM is faster and much more broadly supported by
SMB3 servers).  To make this stronger encryption type, GCM-256,
required (the first and only choice, you would use module parameter
"require_gcm_256."
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Add new module load parameter require_gcm_256. If set, then only
request AES-256-GCM (strongest encryption type).
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

This is basically the same as STATUS_LOGON_FAILURE,
but after the account is locked out.
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Currently there are three supported signing algorithms
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

RHBZ: 1848178

Some calls that set attributes, like utimensat(), are not supposed to return
-EINTR and thus do not have handlers for this in glibc which causes us
to leak -EINTR to the applications which are also unprepared to handle it.

For example tar will break if utimensat() return -EINTR and abort unpacking
the archive. Other applications may break too.

To handle this we add checks, and retry, for -EINTR in cifs_setattr()
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Currently STATUS_IO_TIMEOUT is not treated as retriable error.
It is currently mapped to ETIMEDOUT and returned to userspace
for most system calls. STATUS_IO_TIMEOUT is returned by server
in case of unavailability or throttling errors.

This patch will map the STATUS_IO_TIMEOUT to EAGAIN, so that it
can be retried. Also, added a check to drop the connection to
not overload the server in case of ongoing unavailability.
Signed-off-by: Rohith Surabattula <rohiths@microsoft.com>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Cleanup patch for followon to cache additional information for the root directory
when directory lease held.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Cleanup patch for followon to cache additional information for the root directory
when directory lease held.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

MS-SMB2 was updated recently to include new protocol definitions for
updated compression payload header and new RDMA transform capabilities
Update structure definitions in smb2pdu.h to match
Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>

In encryption capabilities negotiate context can now request
AES256 GCM or CCM
Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>

When converting trailing spaces and periods in paths, do so
for every component of the path, not just the last component.
If the conversion is not done for every path component, then
subsequent operations in directories with trailing spaces or
periods (e.g. create(), mkdir()) will fail with ENOENT. This
is because on the server, the directory will have a special
symbol in its name, and the client needs to provide the same.
Signed-off-by: Boris Protopopov <pboris@amazon.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Merge misc fixes from Andrew Morton:
 "Five fixes.

  Subsystems affected by this patch series: MAINTAINERS, mm/pagemap,
  mm/swap, and mm/hugetlb"

* emailed patches from Andrew Morton <akpm@linux-foundation.org>:
  mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged
  mm: validate inode in mapping_set_error()
  mm: mmap: Fix general protection fault in unlink_file_vma()
  MAINTAINERS: Antoine Tenart's email address
  MAINTAINERS: change hardening mailing list

Pull vfs fix from Al Viro:
 "Fixes an obvious bug (memory leak introduced in 5.8)"

* 'fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  pipe: Fix memory leaks in create_pipe_files()

Pull x86 fixes from Ingo Molnar:
 "Two fixes:

   - Fix a (hopefully final) IRQ state tracking bug vs MCE handling

   - Fix a documentation link"

* tag 'x86-urgent-2020-10-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  Documentation/x86: Fix incorrect references to zero-page.txt
  x86/mce: Use idtentry_nmi_enter/exit()

Pull perf fix from Ingo Molnar:
 "Fix an error handling bug that can cause a lockup if a CPU is offline
  (doh ...)"

* tag 'perf-urgent-2020-10-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  perf: Fix task_function_call() error handling

When memory is hotplug added or removed the min_free_kbytes should be
recalculated based on what is expected by khugepaged.  Currently after
hotplug, min_free_kbytes will be set to a lower default and higher
default set when THP enabled is lost.

This change restores min_free_kbytes as expected for THP consumers.

[vijayb@linux.microsoft.com: v5]
  Link: https://lkml.kernel.org/r/1601398153-5517-1-git-send-email-vijayb@linux.microsoft.com

Fixes: f000565a ("thp: set recommended min free kbytes")
Signed-off-by: Vijay Balakrishna <vijayb@linux.microsoft.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Allen Pais <apais@microsoft.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/1600305709-2319-2-git-send-email-vijayb@linux.microsoft.com
Link: https://lkml.kernel.org/r/1600204258-13683-1-git-send-email-vijayb@linux.microsoft.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

The swap address_space doesn't have host. Thus, it makes kernel crash once
swap write meets error. Fix it.

Fixes: 735e4ae5 ("vfs: track per-sb writeback errors and report them to syncfs")
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Andres Freund <andres@anarazel.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: David Howells <dhowells@redhat.com>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20201010000650.750063-1-minchan@kernel.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

The syzbot reported the below general protection fault:

  general protection fault, probably for non-canonical address
  0xe00eeaee0000003b: 0000 [#1] PREEMPT SMP KASAN
  KASAN: maybe wild-memory-access in range [0x00777770000001d8-0x00777770000001df]
  CPU: 1 PID: 10488 Comm: syz-executor721 Not tainted 5.9.0-rc3-syzkaller #0
  RIP: 0010:unlink_file_vma+0x57/0xb0 mm/mmap.c:164
  Call Trace:
     free_pgtables+0x1b3/0x2f0 mm/memory.c:415
     exit_mmap+0x2c0/0x530 mm/mmap.c:3184
     __mmput+0x122/0x470 kernel/fork.c:1076
     mmput+0x53/0x60 kernel/fork.c:1097
     exit_mm kernel/exit.c:483 [inline]
     do_exit+0xa8b/0x29f0 kernel/exit.c:793
     do_group_exit+0x125/0x310 kernel/exit.c:903
     get_signal+0x428/0x1f00 kernel/signal.c:2757
     arch_do_signal+0x82/0x2520 arch/x86/kernel/signal.c:811
     exit_to_user_mode_loop kernel/entry/common.c:136 [inline]
     exit_to_user_mode_prepare+0x1ae/0x200 kernel/entry/common.c:167
     syscall_exit_to_user_mode+0x7e/0x2e0 kernel/entry/common.c:242
     entry_SYSCALL_64_after_hwframe+0x44/0xa9

It's because the ->mmap() callback can change vma->vm_file and fput the
original file.  But the commit d70cec89 ("mm: mmap: merge vma after
call_mmap() if possible") failed to catch this case and always fput()
the original file, hence add an extra fput().

[ Thanks Hillf for pointing this extra fput() out. ]

Fixes: d70cec89 ("mm: mmap: merge vma after call_mmap() if possible")
Reported-by: syzbot+c5d5a51dcbb558ca0cb5@syzkaller.appspotmail.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Christian König <ckoenig.leichtzumerken@gmail.com>
Cc: Hongxiang Lou <louhongxiang@huawei.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Link: https://lkml.kernel.org/r/20200916090733.31427-1-linmiaohe@huawei.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Use my kernel.org address instead of my bootlin.com one.
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/20201005164533.16811-1-atenart@kernel.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

As more email from git history gets aimed at the OpenWall
kernel-hardening@ list, there has been a desire to separate "new topics"
from "on-going" work.

To handle this, the superset of hardening email topics are now to be
directed to linux-hardening@vger.kernel.org.

Update the MAINTAINERS file and the .mailmap to accomplish this, so that
linux-hardening@ can be treated like any other regular upstream kernel
development list.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Emese Revfy <re.emese@gmail.com>
Cc: "Tobin C. Harding" <me@tobin.cc>
Cc: Tycho Andersen <tycho@tycho.pizza>
Cc: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/linux-hardening/202010051443.279CC265D@keescook/
Link: https://lkml.kernel.org/r/20201006000012.2768958-1-keescook@chromium.orgSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Pull i2c fixes from Wolfram Sang:
 "Some more driver bugfixes for I2C. Including a revert - the updated
  series for it will come during the next merge window"

* 'i2c/for-current' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux:
  i2c: owl: Clear NACK and BUS error bits
  Revert "i2c: imx: Fix reset of I2SR_IAL flag"
  i2c: meson: fixup rate calculation with filter delay
  i2c: meson: keep peripheral clock enabled
  i2c: meson: fix clock setting overwrite
  i2c: imx: Fix reset of I2SR_IAL flag

On setxattr() syscall path due to an apprent typo the size of a dynamically
allocated memory chunk for storing struct smb2_file_full_ea_info object is
computed incorrectly, to be more precise the first addend is the size of
a pointer instead of the wanted object size. Coincidentally it makes no
difference on 64-bit platforms, however on 32-bit targets the following
memcpy() writes 4 bytes of data outside of the dynamically allocated memory.

  =============================================================================
  BUG kmalloc-16 (Not tainted): Redzone overwritten
  -----------------------------------------------------------------------------

  Disabling lock debugging due to kernel taint
  INFO: 0x79e69a6f-0x9e5cdecf @offset=368. First byte 0x73 instead of 0xcc
  INFO: Slab 0xd36d2454 objects=85 used=51 fp=0xf7d0fc7a flags=0x35000201
  INFO: Object 0x6f171df3 @offset=352 fp=0x00000000

  Redzone 5d4ff02d: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc  ................
  Object 6f171df3: 00 00 00 00 00 05 06 00 73 6e 72 75 62 00 66 69  ........snrub.fi
  Redzone 79e69a6f: 73 68 32 0a                                      sh2.
  Padding 56254d82: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
  CPU: 0 PID: 8196 Comm: attr Tainted: G    B             5.9.0-rc8+ #3
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
  Call Trace:
   dump_stack+0x54/0x6e
   print_trailer+0x12c/0x134
   check_bytes_and_report.cold+0x3e/0x69
   check_object+0x18c/0x250
   free_debug_processing+0xfe/0x230
   __slab_free+0x1c0/0x300
   kfree+0x1d3/0x220
   smb2_set_ea+0x27d/0x540
   cifs_xattr_set+0x57f/0x620
   __vfs_setxattr+0x4e/0x60
   __vfs_setxattr_noperm+0x4e/0x100
   __vfs_setxattr_locked+0xae/0xd0
   vfs_setxattr+0x4e/0xe0
   setxattr+0x12c/0x1a0
   path_setxattr+0xa4/0xc0
   __ia32_sys_lsetxattr+0x1d/0x20
   __do_fast_syscall_32+0x40/0x70
   do_fast_syscall_32+0x29/0x60
   do_SYSENTER_32+0x15/0x20
   entry_SYSENTER_32+0x9f/0xf2

Fixes: 5517554e ("cifs: Add support for writing attributes on SMB2+")
Signed-off-by: Vladimir Zapolskiy <vladimir@tuxera.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

There have been elusive reports of filemap_fault() hitting its
VM_BUG_ON_PAGE(page_to_pgoff(page) != offset, page) on kernels built
with CONFIG_READ_ONLY_THP_FOR_FS=y.

Suren has hit it on a kernel with CONFIG_READ_ONLY_THP_FOR_FS=y and
CONFIG_NUMA is not set: and he has analyzed it down to how khugepaged
without NUMA reuses the same huge page after collapse_file() failed
(whereas NUMA targets its allocation to the respective node each time).
And most of us were usually testing with CONFIG_NUMA=y kernels.

collapse_file(old start)
  new_page = khugepaged_alloc_page(hpage)
  __SetPageLocked(new_page)
  new_page->index = start // hpage->index=old offset
  new_page->mapping = mapping
  xas_store(&xas, new_page)

                          filemap_fault
                            page = find_get_page(mapping, offset)
                            // if offset falls inside hpage then
                            // compound_head(page) == hpage
                            lock_page_maybe_drop_mmap()
                              __lock_page(page)

  // collapse fails
  xas_store(&xas, old page)
  new_page->mapping = NULL
  unlock_page(new_page)

collapse_file(new start)
  new_page = khugepaged_alloc_page(hpage)
  __SetPageLocked(new_page)
  new_page->index = start // hpage->index=new offset
  new_page->mapping = mapping // mapping becomes valid again

                            // since compound_head(page) == hpage
                            // page_to_pgoff(page) got changed
                            VM_BUG_ON_PAGE(page_to_pgoff(page) != offset)

An initial patch replaced __SetPageLocked() by lock_page(), which did
fix the race which Suren illustrates above.  But testing showed that it's
not good enough: if the racing task's __lock_page() gets delayed long
after its find_get_page(), then it may follow collapse_file(new start)'s
successful final unlock_page(), and crash on the same VM_BUG_ON_PAGE.

It could be fixed by relaxing filemap_fault()'s VM_BUG_ON_PAGE to a
check and retry (as is done for mapping), with similar relaxations in
find_lock_entry() and pagecache_get_page(): but it's not obvious what
else might get caught out; and khugepaged non-NUMA appears to be unique
in exposing a page to page cache, then revoking, without going through
a full cycle of freeing before reuse.

Instead, non-NUMA khugepaged_prealloc_page() release the old page
if anyone else has a reference to it (1% of cases when I tested).

Although never reported on huge tmpfs, I believe its find_lock_entry()
has been at similar risk; but huge tmpfs does not rely on khugepaged
for its normal working nearly so much as READ_ONLY_THP_FOR_FS does.
Reported-by: Denis Lisov <dennis.lissov@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=206569
Link: https://lore.kernel.org/linux-mm/?q=20200219144635.3b7417145de19b65f258c943%40linux-foundation.orgReported-by: Qian Cai <cai@lca.pw>
Link: https://lore.kernel.org/linux-xfs/?q=20200616013309.GB815%40lca.pwReported-and-analyzed-by: Suren Baghdasaryan <surenb@google.com>
Fixes: 87c460a0 ("mm/khugepaged: collapse_shmem() without freezing new_page")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org # v4.9+
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>