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

[David Hildenbrand]

On 11.08.23 17:03, Peter Xu wrote:
> 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.

Using fork() is the easiest way. mremap(), MADV_DONTNEED, munmap, ...

You might end up having to scan page tables and/or the rmap to figure 
out which mapcount to adjust, which we should absolutely avoid.


> 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;

Sure. But note that it's a moving target, and some discussions have been 
going on for a long time. I recall there were various discussions, 
including LSF/MM, mm biweekly meeting, and more. So even if you scan 
through all that, you might either get outdated or incomplete information.

--
Cheers,

David / dhildenb