[cc linux-fsdevel as a heads-up] On Thu, May 08, 2014 at 08:16:24AM +0900, Namjae Jeon wrote: > When we perform a data integrity sync we tag all the dirty pages with > PAGECACHE_TAG_TOWRITE at start of ext4_da_writepages. > Later we check for this tag in write_cache_pages_da and creates a > struct mpage_da_data containing contiguously indexed pages tagged with this > tag and sync these pages with a call to mpage_da_map_and_submit. > This process is done in while loop until all the PAGECACHE_TAG_TOWRITE pages > are synced. We also do journal start and stop in each iteration. > journal_stop could initiate journal commit which would call ext4_writepage > which in turn will call ext4_bio_write_page even for delayed OR unwritten > buffers. When ext4_bio_write_page is called for such buffers, even though it > does not sync them but it clears the PAGECACHE_TAG_TOWRITE of the corresponding > page and hence these pages are also not synced by the currently running data > integrity sync. We will end up with dirty pages although sync is completed. > > This could cause a potential data loss when the sync call is followed by a > truncate_pagecache call, which is exactly the case in collapse_range. > (It will cause generic/127 failure in xfstests) Yes, this is a patch that went into 3.16, but I only just found out about it because Brian just found a very similar data corruption bug in XFS. i.e. a partial page write was starting writeback and hence clearing PAGECACHE_TAG_TOWRITE before the page was fully cleaned and hence WB_SYNC_ALL wasn't writing the entire page. http://oss.sgi.com/pipermail/xfs/2014-September/038150.html http://oss.sgi.com/pipermail/xfs/2014-September/038167.html IOWs, if a filesystem does write-ahead in ->writepages() or relies on the write_cache_pages() layer to reissue dirty pages in partial page write situations for data integrity purposes, then it needs to be converted to use set_page_writeback_keepwrite() until the page is fully clean, at which point it can then use set_page_writeback(). For everyone: if one filesystem is using the generic code incorrectly, then it is likely the same or similar bugs exist in other filesystems. As a courtesy to your fellow filesystem developers, if you find a data corruption bug caused by interactions with the generic code can the fixes please be CC'd to linux-fsdevel so everyone knows about the issue? This is especially important if new interfaces in the generic code have been added to avoid the problem. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
