Re: [HELP] FUSE writeback performance bottleneck

[Dave Chinner]

On Mon, Jun 03, 2024 at 05:19:44PM +0200, Miklos Szeredi wrote:
> On Mon, 3 Jun 2024 at 16:43, Bernd Schubert <bernd.schubert@xxxxxxxxxxx> 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).

I thought we had PR_SET_IO_FLUSHER for that. Requires
CAP_SYS_RESOURCES but no other privileges, then the userspace
server will then always operate in PF_MEMALLOC_NOIO |
PF_LOCAL_THROTTLE memory allocation context.

-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx