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Date:   Fri, 23 Apr 2021 10:40:38 -0400
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Jan Kara <jack@suse.cz>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Zhang Yi <yi.zhang@huawei.com>, linux-ext4@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, adilger.kernel@dilger.ca,
        yukuai3@huawei.com
Subject: Re: [RFC PATCH v2 7/7] ext4: fix race between blkdev_releasepage()
 and ext4_put_super()
Message-ID: <YILcZpQh8T//6HLb@mit.edu>
References: <20210414134737.2366971-1-yi.zhang@huawei.com>
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 <20210420130841.GA3618564@infradead.org>
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 <20210422090410.GA26221@quack2.suse.cz>
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On Thu, Apr 22, 2021 at 11:04:11AM +0200, Jan Kara wrote:
> Yes, I understand that. What I was more asking about is: Does it really
> matter we leave those buffer heads and journal heads unreclaimed. I
> understand it could be triggering premature OOM in theory but is it a
> problem in practice? Was there some observed practical case for which this
> was added or was it just added due to the theoretical concern?

I was doing some research, and found the mail thread which inspired
bdev_try_to_free_page():

https://lore.kernel.org/linux-ext4/20081202200647.72cc5807.toshi.okajima@jp.fujitsu.com/

From what I can tell Toshi Okajima did have a test workload which
would trigger blkdev_releasepage().  He didn't specify it in the mail
thread as near as I can tell, but he did use it to test the page.
Thinking about it, it shouldn't be hard to trigger it via something like:

   find /mnt -print0 | xargs -0 touch

in a memory contrained box with a large file system attached (a
bookshelf NAS scenario).  Under the right circumstances, I'm pretty
sure a premature OOM could be demonstrated.

					- Ted