linux-kernel - Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back dirty pages

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Message-ID: <CAJfpegsurP8JshxFah0vCwBQicc0ijRnGyLeZZ-4tio6BHqEzQ@mail.gmail.com>
Date:   Wed, 14 Apr 2021 11:02:31 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     Peng Tao <tao.peng@...ux.alibaba.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back
 dirty pages

On Wed, Apr 14, 2021 at 10:42 AM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:

> Sorry I missed this patch before, and I've tested this patch, it seems
> can solve the deadlock issue I met before.

Great, thanks for testing.

> But look at this patch in detail, I think this patch only reduced the
> deadlock window, but did not remove the possible deadlock scenario
> completely like I explained in the commit log.
>
> Since the fuse_fill_write_pages() can still lock the partitail page in
> your patch, and will be wait for the partitail page waritehack is
> completed if writeback is set in fuse_send_write_pages().
>
> But at the same time, a writeback worker thread may be waiting for
> trying to lock the partitail page to write a bunch of dirty pages by
> fuse_writepages().

As you say, fuse_fill_write_pages() will lock a partial page.  This
page cannot become dirty, only after being read completely, which
first requires the page lock.  So dirtying this page can only happen
after the writeback of the fragment was completed.

I don't see how this could lead to a deadlock.

Thanks,
Miklos