3.16.64-rc1 review patch. If anyone has any objections, please let me know.
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From: Brian Foster bfoster@redhat.com
commit 04197b341f23b908193308b8d63d17ff23232598 upstream.
We've had reports of generic/095 causing XFS to BUG() in __xfs_get_blocks() due to the existence of delalloc blocks on a direct I/O read. generic/095 issues a mix of various types of I/O, including direct and memory mapped I/O to a single file. This is clearly not supported behavior and is known to lead to such problems. E.g., the lack of exclusion between the direct I/O and write fault paths means that a write fault can allocate delalloc blocks in a region of a file that was previously a hole after the direct read has attempted to flush/inval the file range, but before it actually reads the block mapping. In turn, the direct read discovers a delalloc extent and cannot proceed.
While the appropriate solution here is to not mix direct and memory mapped I/O to the same regions of the same file, the current BUG_ON() behavior is probably overkill as it can crash the entire system. Instead, localize the failure to the I/O in question by returning an error for a direct I/O that cannot be handled safely due to delalloc blocks. Be careful to allow the case of a direct write to post-eof delalloc blocks. This can occur due to speculative preallocation and is safe as post-eof blocks are not accompanied by dirty pages in pagecache (conversely, preallocation within eof must have been zeroed, and thus dirtied, before the inode size could have been increased beyond said blocks).
Finally, provide an additional warning if a direct I/O write occurs while the file is memory mapped. This may not catch all problematic scenarios, but provides a hint that some known-to-be-problematic I/O methods are in use.
Signed-off-by: Brian Foster bfoster@redhat.com Reviewed-by: Dave Chinner dchinner@redhat.com Signed-off-by: Dave Chinner david@fromorbit.com [bwh: Backported to 3.16: - Assign positive error code to error variable - Adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- fs/xfs/xfs_aops.c | 22 ++++++++++++++++++++-- 1 file changed, 20 insertions(+), 2 deletions(-)
--- a/fs/xfs/xfs_aops.c +++ b/fs/xfs/xfs_aops.c @@ -1300,6 +1300,26 @@ __xfs_get_blocks( if (error) goto out_unlock;
+ /* + * The only time we can ever safely find delalloc blocks on direct I/O + * is a dio write to post-eof speculative preallocation. All other + * scenarios are indicative of a problem or misuse (such as mixing + * direct and mapped I/O). + * + * The file may be unmapped by the time we get here so we cannot + * reliably fail the I/O based on mapping. Instead, fail the I/O if this + * is a read or a write within eof. Otherwise, carry on but warn as a + * precuation if the file happens to be mapped. + */ + if (direct && imap.br_startblock == DELAYSTARTBLOCK) { + if (!create || offset < i_size_read(VFS_I(ip))) { + WARN_ON_ONCE(1); + error = EIO; + goto out_unlock; + } + WARN_ON_ONCE(mapping_mapped(VFS_I(ip)->i_mapping)); + } + if (create && (!nimaps || (imap.br_startblock == HOLESTARTBLOCK || @@ -1383,7 +1403,6 @@ __xfs_get_blocks( set_buffer_new(bh_result);
if (imap.br_startblock == DELAYSTARTBLOCK) { - BUG_ON(direct); if (create) { set_buffer_uptodate(bh_result); set_buffer_mapped(bh_result);