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

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Jun 16, 2021 at 06:48:34PM +0200, Vlastimil Babka wrote:
> On 6/16/21 3:47 PM, Miklos Szeredi wrote:
> > On Wed, 16 Jun 2021 at 14:59, Matthew Wilcox <willy@infradead.org> wrote:
> >>
> >> > 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
> 
> "working set" is the notion of the set of pages that the workload needs to
> access at the moment/relatively short time window, and it would be beneficial if
> all of it could fit in the RAM.
> PG_workinsgset is part of the mechanism that tries to estimate this ideal set of
> pages, and especially when the workload shifts to another set of pages, in order
> to guide reclaim better. See the big comment at the top of mm/workingset.c for
> details
> 
> > swap?  If so, how can such a page be part of a file mapping?
> 
> Not related to swap. It was actually first implemented only for file pages (page
> cache), but then extended to anonymous pages by aae466b0052e ("mm/swap:
> implement workingset detection for anonymous LRU")

Thanks, yes.

Think of it as similar to PG_active. It's just another usage/heat
indicator of file and anon pages on the reclaim LRU that, unlike
PG_active, persists across deactivation and even reclaim (we store it
in the page cache / swapper cache tree until the page refaults).

So if fuse accepts pages that can legally have PG_active set,
PG_workingset is fine too.