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From: Christoph Hellwig <hch@infradead.org>
To: Zhang Yi <yi.zhang@huawei.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	tytso@mit.edu, adilger.kernel@dilger.ca, jack@suse.cz,
	yukuai3@huawei.com
Subject: Re: [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage() and ext4_put_super()
Date: Tue, 20 Apr 2021 14:08:41 +0100
Message-ID: <20210420130841.GA3618564@infradead.org> (raw)
In-Reply-To: <ca810e21-5f92-ee6c-a046-255c70c6bf78@huawei.com>

On Fri, Apr 16, 2021 at 04:00:48PM +0800, Zhang Yi wrote:
> Now, we use already use "if (bdev->bd_super)" to prevent call into
> ->bdev_try_to_free_page unless the super is alive, and the problem is
> bd_super becomes NULL concurrently after this check. So, IIUC, I think it's
> the same to switch to check the superblock is active or not. The acvive
> flag also could becomes inactive (raced by umount) after we call into
> bdev_try_to_free_page().

Indeed.

> In order to close this race, One solution is introduce a lock to synchronize
> the active state between kill_block_super() and blkdev_releasepage(), but
> the releasing page process have to try to acquire this lock in
> blkdev_releasepage() for each page, and the umount process still need to wait
> until the page release if some one invoke into ->bdev_try_to_free_page().
> I think this solution may affect performace and is not a good way.
> Think about it in depth, use percpu refcount seems have the smallest
> performance effect on blkdev_releasepage().
> 
> If you don't like the refcount, maybe we could add synchronize_rcu_expedited()
> in ext4_put_super(), it also could prevent this race. Any suggestions?

I really don't like to put a lot of overhead into the core VFS and block
device code.  ext4/jbd does not own the block device inode and really
has no business controlling releasepage for it.  I suspect the right
answer might be to simply revert the commit that added this hook.

  reply index

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-14 13:47 [RFC PATCH v2 0/7] ext4, jbd2: fix 3 issues about bdev_try_to_free_page() Zhang Yi
2021-04-14 13:47 ` [RFC PATCH v2 1/7] jbd2: remove the out label in __jbd2_journal_remove_checkpoint() Zhang Yi
2021-04-21 10:01   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 2/7] jbd2: ensure abort the journal if detect IO error when writing original buffer back Zhang Yi
2021-04-21 13:20   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 3/7] jbd2: don't abort the journal when freeing buffers Zhang Yi
2021-04-21 13:23   ` Jan Kara
2021-04-14 13:47 ` [RFC PATCH v2 4/7] jbd2: do not free buffers in jbd2_journal_try_to_free_buffers() Zhang Yi
2021-04-15 14:46   ` Christoph Hellwig
2021-04-14 13:47 ` [RFC PATCH v2 5/7] ext4: use RCU to protect accessing superblock in blkdev_releasepage() Zhang Yi
2021-04-15 14:48   ` Christoph Hellwig
2021-04-14 13:47 ` [RFC PATCH v2 6/7] fs: introduce a usage count into the superblock Zhang Yi
2021-04-15 14:40   ` Christoph Hellwig
2021-04-16  8:00     ` Zhang Yi
2021-04-14 13:47 ` [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage() and ext4_put_super() Zhang Yi
2021-04-15 14:52   ` Christoph Hellwig
2021-04-16  8:00     ` Zhang Yi
2021-04-20 13:08       ` Christoph Hellwig [this message]
2021-04-21 13:46         ` Jan Kara
2021-04-21 16:57           ` Theodore Ts'o
2021-04-22  9:04             ` Jan Kara
2021-04-23 11:39               ` Zhang Yi
2021-04-23 16:06                 ` Jan Kara
2021-04-23 14:40               ` Theodore Ts'o

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