Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

Peter Xu
2023-08-11T15:04:13Z

On Thu, Aug 10, 2023 at 11:59:25PM +0200, David Hildenbrand wrote:
> On 10.08.23 23:54, Matthew Wilcox wrote:
> > On Thu, Aug 10, 2023 at 05:48:19PM -0400, Peter Xu wrote:
> > > > Yes, that comment from Hugh primarily discusses how we could possibly
> > > > optimize the loop, and if relying on folio_nr_pages_mapped() to reduce the
> > > > iterations would be racy. As far as I can see, there are cases where "it
> > > > would be certainly a bad idea" :)
> > >
> > > Is the race described about mapcount being changed right after it's read?
> > > Are you aware of anything specific that will be broken, and will be fixed
> > > with this patch?
> >
> > The problem is that people check the mapcount while holding no locks;
> > not the PTL, not the page lock. So it's an unfixable race.
> >
> > > Having a total mapcount does sound helpful if partial folio is common
> > > indeed.
> > >
> > > I'm curious whether that'll be so common after the large anon folio work -
> > > isn't it be sad if partial folio will be a norm? It sounds to me that's
> > > the case when small page sizes should be used.. and it's prone to waste?
> >
> > The problem is that entire_mapcount isn't really entire_mapcount.
> > It's pmd_mapcount. I have had thoughts about using it as entire_mapcount,
> > but it gets gnarly when people do partial unmaps. So the _usual_ case
> > ends up touching every struct page. Which sucks. Also it's one of the
> > things which stands in the way of shrinking struct page.
>
> Right, so one current idea is to have a single total_mapcount and look into
> removing the subpage mapcounts (which will require first removing
> _nr_pages_mapped, because that's still one of the important users).
>
> Until we get there, also rmap code has to do eventually "more tracking" and
> might, unfortunately, end up slower.
>
> >
> > But it's kind of annoying to explain all of this to you individually.
> > There have been hundreds of emails about it over the last months on
> > this mailing list. It would be nice if you could catch up instead of
> > jumping in.
>
> To be fair, a lot of the details are not readily available and in the heads
> of selected people :)
>
> Peter, if you're interested, we can discuss the current plans, issues and
> ideas offline!

Thanks for offering help, David.

Personally I still am unclear yet on why entire_mapcount cannot be used as
full-folio mapcounts, and why "partial unmap" can happen a lot (I don't
expect), but yeah I can try to catch up to educate myself first.

The only issue regarding an offline sync-up is that even if David will help
Peter on catching up the bits, it'll not scale when another Peter2 had the
same question.. So David, rather than I waste your time on helping one
person, let me try to catch up with the public threads - I'm not sure how
far I can go myself; otoh thread links will definitely be helpful to be
replied here, so anyone else can reference too. I collected a list (which
can be enriched) of few threads that might be related, just in case helpful
to anyone besides myself:

[PATCH 0/2] don't use mapcount() to check large folio sharing
https://lore.kernel.org/r/20230728161356.1784568-1-fengwei.yin@intel.com

[PATCH v1-v2 0/3] support large folio for mlock
https://lore.kernel.org/r/20230728070929.2487065-1-fengwei.yin@intel.com
https://lore.kernel.org/r/20230809061105.3369958-1-fengwei.yin@intel.com

[PATCH v1 0/4] Optimize mmap_exit for large folios
https://lore.kernel.org/r/20230810103332.3062143-1-ryan.roberts@arm.com

[PATCH v4-v5 0/5] variable-order, large folios for anonymous memory
https://lore.kernel.org/linux-mm/20230726095146.2826796-1-ryan.roberts@arm.com/
https://lore.kernel.org/r/20230810142942.3169679-1-ryan.roberts@arm.com

[PATCH v3-v4 0/3] Optimize large folio interaction with deferred split
(I assumed Ryan's this one goes into the previous set v5 finally, so just
the discussions as reference)
https://lore.kernel.org/r/20230720112955.643283-1-ryan.roberts@arm.com
https://lore.kernel.org/r/20230727141837.3386072-1-ryan.roberts@arm.com

[RFC PATCH v2 0/4] fix large folio for madvise_cold_or_pageout()
https://lore.kernel.org/r/20230721094043.2506691-1-fengwei.yin@intel.com

I'm not sure how far I'll go; maybe I'll start working on something else
before I finish all of them. I'll see..

Not allowing people to jump in will definitely cause less interactions and
less involvement/open-ness for the mm community, as sometimes people can't
easily judge when it's proper to jump in.

IMHO the ideal solution is always keep all discussions public (either
meetings with recordings, or shared online documents, always use on-list
discussions, etc.), then share the links.

--
Peter Xu