ext4: fix races between page faults and hole punching

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Currently, page faults and hole punching are completely unsynchronized.
This can result in page fault faulting in a page into a range that we
are punching after truncate_pagecache_range() has been called and thus
we can end up with a page mapped to disk blocks that will be shortly
freed. Filesystem corruption will shortly follow. Note that the same
race is avoided for truncate by checking page fault offset against
i_size but there isn't similar mechanism available for punching holes.

Fix the problem by creating new rw semaphore i_mmap_sem in inode and
grab it for writing over truncate, hole punching, and other functions
removing blocks from extent tree and for read over page faults. We
cannot easily use i_data_sem for this since that ranks below transaction
start and we need something ranking above it so that it can be held over
the whole truncate / hole punching operation. Also remove various
workarounds we had in the code to reduce race window when page fault
could have created pages with stale mapping information.

Signed-off-by: Jan Kara <jack@suse.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

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This commit is contained in:

Jan Kara 2015-12-07 14:28:03 -05:00 • committed by Theodore Ts'o

parent f41683a204

commit ea3d7209ca

6 changed files with 127 additions and 42 deletions

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10

fs/ext4/ext4.h

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54

fs/ext4/extents.c

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66

fs/ext4/file.c

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36

fs/ext4/inode.c

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1

fs/ext4/super.c

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2

fs/ext4/truncate.h

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