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From:   Khazhy Kumykov <khazhy@google.com>
Date:   Fri, 18 Mar 2022 18:45:04 -0700
Message-ID: <CACGdZYJGm2VP0wHhqCegp+P0rKpEhdTdvjWK_T3+9FNHM3W8cw@mail.gmail.com>
Subject: Re: [PATCH RFC] nfsd: avoid recursive locking through fsnotify
To:     Trond Myklebust <trondmy@hammerspace.com>
Cc:     "bfields@fieldses.org" <bfields@fieldses.org>,
        "chuck.lever@oracle.com" <chuck.lever@oracle.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, Mar 18, 2022 at 5:36 PM Trond Myklebust <trondmy@hammerspace.com> wrote:
>
> Isn't that stack trace showing a slab direct reclaim, and not a
> filesystem writeback situation?
>
> Does memalloc_nofs_save()/restore() really fix this problem? It seems
> to me that it cannot, particularly since knfsd is not a filesystem, and
> so does not ever handle writeback of dirty pages.

Ah you're right. An alternative would be delaying the destroy_mark,
which I am noticing now that, on mainline, the shrinker calls
nfsd_file_dispose_list_delayed, something missing from the version of
5.4 I was looking at.

To my reading 9542e6a643fc6 ("nfsd: Containerise filecache
laundrette") should have the effect of fixing this deadlock (and the
message does explicitly call out notifier deadlocks) - maybe something
to send towards stable?
>
>
> Cc: the linux-mm mailing list in search of answers to the above 2
> questions.
>
> --
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
>
>

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