Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

[David Hildenbrand]

On 02.08.23 13:20, Ryan Roberts wrote:
> On 02/08/2023 11:48, David Hildenbrand wrote:
> > On 02.08.23 12:27, Ryan Roberts wrote:
> > > On 28/07/2023 17:13, Yin Fengwei wrote:
> > > > In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
> > > > folio_mapcount() is used to check whether the folio is shared. But it's
> > > > not correct as folio_mapcount() returns total mapcount of large folio.
> > > >
> > > > Use folio_estimated_sharers() here as the estimated number is enough.
> > > >
> > > > Yin Fengwei (2):
> > > >     madvise: don't use mapcount() against large folio for sharing check
> > > >     madvise: don't use mapcount() against large folio for sharing check
> > > >
> > > >    mm/huge_memory.c | 2 +-
> > > >    mm/madvise.c     | 6 +++---
> > > >    2 files changed, 4 insertions(+), 4 deletions(-)
> > >
> > > As a set of fixes, I agree this is definitely an improvement, so:
> > >
> > > Reviewed-By: Ryan Roberts
> > >
> > >
> > > But I have a couple of comments around further improvements;
> > >
> > > Once we have the scheme that David is working on to be able to provide precise
> > > exclusive vs shared info, we will probably want to move to that. Although that
> > > scheme will need access to the mm_struct of a process known to be mapping the
> >
> > There are probably ways to work around lack of mm_struct, but it would not be
> > completely for free. But passing the mm_struct should probably be an easy
> > refactoring.
> >
> > > folio. We have that info, but its not passed to folio_estimated_sharers() so we
> > > can't just reimplement folio_estimated_sharers() - we will need to rework these
> > > call sites again.
> >
> > We should probably just have a
> >
> > folio_maybe_mapped_shared()
> >
> > with proper documentation. Nobody should care about the exact number.
> >
> >
> > If my scheme for anon pages makes it in, that would be precise for anon pages
> > and we could document that. Once we can handle pagecache pages as well to get a
> > precise answer, we could change to folio_mapped_shared() and adjust the
> > documentation.
>
> Makes sense to me. I'm assuming your change would allow us to get rid of
> PG_anon_exclusive too? In which case we would also want a precise API
> specifically for anon folios for the CoW case, without waiting for pagecache
> page support.

Not necessarily and I'm currently not planning that

On the COW path, I'm planning on using it only when PG_anon_exclusive is 
clear for a compound page, combined with a check that there are no other 
page references besides from mappings: all mappings from me and #refs == 
#mappings -> reuse (set PG_anon_exclusive). That keeps the default (no 
fork) as fast as possible and simple.

> > I just saw
> >
> > https://lkml.kernel.org/r/20230802095346.87449-1-wangkefeng.wang@xxxxxxxxxx
> >
> > that converts a lot of code to folio_estimated_sharers().
> >
> >
> > That patchset, for example, also does
> >
> > total_mapcount(page) > 1 -> folio_estimated_sharers(folio) > 1
> >
> > I'm not 100% sure what to think about that at this point. We eventually add
> > false negatives (actually shared but we fail to detect it) all over the place,
> > instead of having false positives (actually exclusive, but we fail to detect it).
> >
> > And that patch set doesn't even spell that out.
> >
> >
> > Maybe it's as good as we will get, especially if my scheme doesn't make it in.
>
> I've been working on the assumption that your scheme is plan A, and I'm waiting
> for it to unblock forward progress on large anon folios. Is this the right
> approach, or do you think your scheme is sufficiently riskly and/or far out that
> I should aim not to depend on it?

It is plan A. IMHO, it does not feel too risky and/or far out at this 
point -- and the implementation should not end up too complicated. But 
as always, I cannot promise anything before it's been implemented and 
discussed upstream.

Hopefully, we know more soon. I'll get at implementing it fairly soon.

--
Cheers,

David / dhildenb