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

[David Hildenbrand]

On 02.08.23 13:51, Ryan Roberts wrote:
> On 02/08/2023 12:36, David Hildenbrand wrote:
> > 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.
>
> OK, good we are on the same folio... (stolen from Hugh; if a joke is worth
> telling once, its worth telling 1000 times ;-)

Heard it first the time :))

--
Cheers,

David / dhildenb