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From: Miklos Szeredi <miklos@szeredi.hu>
Date: Mon, 3 Jun 2024 17:19:44 +0200
Message-ID: <CAJfpeguts=V9KkBsMJN_WfdkLHPzB6RswGvumVHUMJ87zOAbDQ@mail.gmail.com>
Subject: Re: [HELP] FUSE writeback performance bottleneck
To: Bernd Schubert <bernd.schubert@fastmail.fm>
Cc: Jingbo Xu <jefflexu@linux.alibaba.com>, 
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, lege.wang@jaguarmicro.com
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On Mon, 3 Jun 2024 at 16:43, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
>
>
>
> On 6/3/24 08:17, Jingbo Xu wrote:
> > Hi, Miklos,
> >
> > We spotted a performance bottleneck for FUSE writeback in which the
> > writeback kworker has consumed nearly 100% CPU, among which 40% CPU is
> > used for copy_page().
> >
> > fuse_writepages_fill
> >   alloc tmp_page
> >   copy_highpage
> >
> > This is because of FUSE writeback design (see commit 3be5a52b30aa
> > ("fuse: support writable mmap")), which newly allocates a temp page for
> > each dirty page to be written back, copy content of dirty page to temp
> > page, and then write back the temp page instead.  This special design is
> > intentional to avoid potential deadlocked due to buggy or even malicious
> > fuse user daemon.
>
> I also noticed that and I admin that I don't understand it yet. The commit says
>
> <quote>
>     The basic problem is that there can be no guarantee about the time in which
>     the userspace filesystem will complete a write.  It may be buggy or even
>     malicious, and fail to complete WRITE requests.  We don't want unrelated parts
>     of the system to grind to a halt in such cases.
> </quote>
>
>
> Timing - NFS/cifs/etc have the same issue? Even a local file system has no guarantees
> how fast storage is?

I don't have the details but it boils down to the fact that the
allocation context provided by GFP_NOFS (PF_MEMALLOC_NOFS) cannot be
used by the unprivileged userspace server (and even if it could,
there's no guarantee, that it would).

When this mechanism was introduced, the deadlock was a real
possibility.  I'm not sure that it can still happen, but proving that
it cannot might be difficult.

Thanks,
Miklos