Re: Possible bogus "fuse: trying to steal weird page" warning related to PG_workingset.

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 16 Jun 2021 at 14:59, Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Jun 16, 2021 at 02:31:32PM +0200, Miklos Szeredi wrote:
> > On Mon, 14 Jun 2021 at 11:56, Thomas Lindroth <thomas.lindroth@gmail.com> wrote:
> > >
> > > Hi. I recently upgraded to kernel series 5.10 from 4.19 and I now get warnings like
> > > this in dmesg:
> > >
> > > page:00000000e966ec4e refcount:1 mapcount:0 mapping:0000000000000000 index:0xd3414 pfn:0x14914a
> > > flags: 0x8000000000000077(locked|referenced|uptodate|lru|active|workingset)
> > > raw: 8000000000000077 ffffdc7f4d312b48 ffffdc7f452452c8 0000000000000000
> > > raw: 00000000000d3414 0000000000000000 00000001ffffffff ffff8fd080123000
> > > page dumped because: fuse: trying to steal weird page
> > >
> > > The warning in fuse_check_page() doesn't check for PG_workingset which seems to be what
> > > trips the warning. I'm not entirely sure this is a bogus warning but there used to be
> > > similar bogus warnings caused by a missing PG_waiters check. The PG_workingset
> > > page flag was introduced in 4.20 which explains why I get the warning now.
> > >
> > > I only get the new warning if I do writes to a fuse fs (mergerfs) and at the same
> > > time put the system under memory pressure by running many qemu VMs.
> >
> > AFAICT fuse is trying to steal a pagecache page from a pipe buffer
> > created by splice(2).    The page looks okay, but I have no idea what
> > PG_workingset means in this context.
> >
> > Matthew, can you please help?
>
> PG_workingset was introduced by Johannes:
>
>     mm: workingset: tell cache transitions from workingset thrashing
>
>     Refaults happen during transitions between workingsets as well as in-place
>     thrashing.  Knowing the difference between the two has a range of
>     applications, including measuring the impact of memory shortage on the
>     system performance, as well as the ability to smarter balance pressure
>     between the filesystem cache and the swap-backed workingset.
>
>     During workingset transitions, inactive cache refaults and pushes out
>     established active cache.  When that active cache isn't stale, however,
>     and also ends up refaulting, that's bonafide thrashing.
>
>     Introduce a new page flag that tells on eviction whether the page has been
>     active or not in its lifetime.  This bit is then stored in the shadow
>     entry, to classify refaults as transitioning or thrashing.
>
> so I think it's fine for you to ignore when stealing a page.

I have problem understanding what a workingset is.  Is it related to
swap?  If so, how can such a page be part of a file mapping?

Thanks,
Miklos