linux-ext4 - Re: [PATCH 1/6] jbd2: Issue cache flush after checkpointing even with internal journal

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Date:	Wed, 11 Jan 2012 13:49:18 +0100
From:	Jan Kara <jack@...e.cz>
To:	Ted Tso <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH 1/6] jbd2: Issue cache flush after checkpointing even
 with internal journal

On Wed 11-01-12 01:31:07, Jan Kara wrote:
> When we reach jbd2_cleanup_journal_tail(), there is no guarantee that
> checkpointed buffers are on a stable storage - especially if buffers were
> written out by jbd2_log_do_checkpoint(), they are likely to be only in disk's
> caches. Thus when we update journal superblock effectively removing old
> transaction from journal, this write of superblock can get to stable storage
> before those checkpointed buffers which can result in filesystem corruption
> after a crash.
> 
> A similar problem can happen if we replay the journal and wipe it before
> flushing disk's caches.
> 
> Thus we must unconditionally issue a cache flush before we update journal
> superblock in these cases. The fix is slightly complicated by the fact that we
> have to get log tail before we issue cache flush but we can store it in the
> journal superblock only after the cache flush. Otherwise we risk races where
> new tail is written before appropriate cache flush is finished.
  I found two mostly minor issues in the patch. Anyway, attached is a new
version.

								Honza

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR

View attachment "0001-jbd2-Issue-cache-flush-after-checkpointing-even-with.patch" of type "text/x-patch" (9413 bytes)