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

[David Hildenbrand]

On 02.08.23 14:40, Ryan Roberts wrote:
> On 02/08/2023 13:35, Yin, Fengwei wrote:
> > On 8/2/2023 6:27 PM, 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
> > Thanks.
> >
> > > 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
> > > 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.
> > Yes. This could be extra work. Maybe should delay till David's work is done.
>
> What you have is definitely an improvement over what was there before. And is
> probably the best we can do without David's scheme. So I wouldn't delay this.
> Just pointing out that we will be able to make it even better later on (if
> David's stuff goes in).

Agreed, we just should be careful and clearly spell out the implications 
and that this is eventually also not what we 100% want.

That MADV_PAGEOUT now fails on a PTE-mapped THP -- as can be seen when 
executing the cow selftest where MADV_PAGEOUT will essentially fail -- 
is certainly undesired and should be fixed IMHO.

--
Cheers,

David / dhildenb