linux-ext4 - Re: [RFC PATCH v4 5/8] jbd2,ext4: add a shrinker to release checkpointed buffers

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Message-ID: <YNSdD4sW5ajlk/Cv@mit.edu>
Date:   Thu, 24 Jun 2021 10:56:15 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Zhang Yi <yi.zhang@...wei.com>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        jack@...e.cz, adilger.kernel@...ger.ca, david@...morbit.com,
        hch@...radead.org
Subject: Re: [RFC PATCH v4 5/8] jbd2,ext4: add a shrinker to release
 checkpointed buffers

On Thu, Jun 10, 2021 at 07:24:37PM +0800, Zhang Yi wrote:
> Current metadata buffer release logic in bdev_try_to_free_page() have
> a lot of use-after-free issues when umount filesystem concurrently, and
> it is difficult to fix directly because ext4 is the only user of
> s_op->bdev_try_to_free_page callback and we may have to add more special
> refcount or lock that is only used by ext4 into the common vfs layer,
> which is unacceptable.
> 
> One better solution is remove the bdev_try_to_free_page callback, but
> the real problem is we cannot easily release journal_head on the
> checkpointed buffer, so try_to_free_buffers() cannot release buffers and
> page under memory pressure, which is more likely to trigger
> out-of-memory. So we cannot remove the callback directly before we find
> another way to release journal_head.
> 
> This patch introduce a shrinker to free journal_head on the checkpointed
> transaction. After the journal_head got freed, try_to_free_buffers()
> could free buffer properly.
> 
> Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
> Suggested-by: Jan Kara <jack@...e.cz>

Applied, thanks.

					- Ted