• Matthew Wilcox's avatar
    mm: fix XIP fault vs truncate race · 283307c7
    Matthew Wilcox authored
    DAX is a replacement for the variation of XIP currently supported by the
    ext2 filesystem.  We have three different things in the tree called 'XIP',
    and the new focus is on access to data rather than executables, so a name
    change was in order.  DAX stands for Direct Access.  The X is for
    eXciting.
    
    The new focus on data access has resulted in more careful attention to
    races that exist in the current XIP code, but are not hit by the use-case
    that it was designed for.  XIP's architecture worked fine for ext2, but
    DAX is architected to work with modern filsystems such as ext4 and XFS.
    DAX is not intended for use with btrfs; the value that btrfs adds relies
    on manipulating data and writing data to different locations, while DAX's
    value is for write-in-place and keeping the kernel from touching the data.
    
    DAX was developed in order to support NV-DIMMs, but it's become clear that
    its usefuless extends beyond NV-DIMMs and there are several potential
    customers including the tracing machinery.  Other people want to place the
    kernel log in an area of memory, as long as they have a BIOS that does not
    clear DRAM on reboot.
    
    Patch 1 is a bug fix, probably worth including in 3.18.
    
    Patches 2 & 3 are infrastructure for DAX.
    
    Patches 4-8 replace the XIP code with its DAX equivalents, transforming
    ext2 to use the DAX code as we go.  Note that patch 10 is the
    Documentation patch.
    
    Patches 9-15 clean up after the XIP code, removing the infrastructure
    that is no longer needed and renaming various XIP things to DAX.
    Most of these patches were added after Jan found things he didn't
    like in an earlier version of the ext4 patch ... that had been copied
    from ext2.  So ext2 i being transformed to do things the same way that
    ext4 will later.  The ability to mount ext2 filesystems with the 'xip'
    option is retained, although the 'dax' option is now preferred.
    
    Patch 16 adds some DAX infrastructure to support ext4.
    
    Patch 17 adds DAX support to ext4.  It is broadly similar to ext2's DAX
    support, but it is more efficient than ext4's due to its support for
    unwritten extents.
    
    Patch 18 is another cleanup patch renaming XIP to DAX.
    
    My thanks to Mathieu Desnoyers for his reviews of the v11 patchset.  Most
    of the changes below were based on his feedback.
    
    This patch (of 18):
    
    Pagecache faults recheck i_size after taking the page lock to ensure that
    the fault didn't race against a truncate.  We don't have a page to lock in
    the XIP case, so use i_mmap_lock_read() instead.  It is locked in the
    truncate path in unmap_mapping_range() after updating i_size.  So while we
    hold it in the fault path, we are guaranteed that either i_size has
    already been updated in the truncate path, or that the truncate will
    subsequently call zap_page_range_single() and so remove the mapping we
    have just inserted.
    
    There is a window of time in which i_size has been reduced and the thread
    has a mapping to a page which will be removed from the file, but this is
    harmless as the page will not be allocated to a different purpose before
    the thread's access to it is revoked.
    
    [akpm@linux-foundation.org: switch to i_mmap_lock_read(), add comment in unmap_single_vma()]
    Signed-off-by: default avatarMatthew Wilcox <matthew.r.wilcox@intel.com>
    Reviewed-by: default avatarJan Kara <jack@suse.cz>
    Acked-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Reviewed-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: Andreas Dilger <andreas.dilger@intel.com>
    Cc: Boaz Harrosh <boaz@plexistor.com>
    Cc: Christoph Hellwig <hch@lst.de>
    Cc: Dave Chinner <david@fromorbit.com>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Randy Dunlap <rdunlap@infradead.org>
    Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
    Cc: Theodore Ts'o <tytso@mit.edu>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    283307c7
memory.c 99.7 KB