Re: [PATCH RFC 5/6] mm: rmap: weaken the WARN_ON in __folio_add_anon_rmap()

[Chris Li <chrisl@kernel.org>]

On Mon, Jan 29, 2024 at 2:07 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 29.01.24 04:25, Chris Li wrote:
> > Hi David and Barry,
> >
> > On Mon, Jan 22, 2024 at 10:49 PM Barry Song <21cnbao@gmail.com> wrote:
> >>
> >>>
> >>>
> >>> I have on my todo list to move all that !anon handling out of
> >>> folio_add_anon_rmap_ptes(), and instead make swapin code call add
> >>> folio_add_new_anon_rmap(), where we'll have to pass an exclusive flag
> >>> then (-> whole new folio exclusive).
> >>>
> >>> That's the cleaner approach.
> >>>
> >>
> >> one tricky thing is that sometimes it is hard to know who is the first
> >> one to add rmap and thus should
> >> call folio_add_new_anon_rmap.
> >> especially when we want to support swapin_readahead(), the one who
> >> allocated large filio might not
> >> be that one who firstly does rmap.
> >
> > I think Barry has a point. Two tasks might race to swap in the folio
> > then race to perform the rmap.
> > folio_add_new_anon_rmap() should only call a folio that is absolutely
> > "new", not shared. The sharing in swap cache disqualifies that
> > condition.
>
> We have to hold the folio lock. So only one task at a time might do the
> folio_add_anon_rmap_ptes() right now, and the
> folio_add_new_shared_anon_rmap() in the future [below].
>

Ah, I see. The folio_lock() is the answer I am looking for.

> Also observe how folio_add_anon_rmap_ptes() states that one must hold
> the page lock, because otherwise this would all be completely racy.
>
>  From the pte swp exclusive flags, we know for sure whether we are
> dealing with exclusive vs. shared. I think patch #6 does not properly
> check that all entries are actually the same in that regard (all
> exclusive vs all shared). That likely needs fixing.
>
> [I have converting per-page PageAnonExclusive flags to a single
> per-folio flag on my todo list. I suspect that we'll keep the
> per-swp-pte exlusive bits, but the question is rather what we can
> actually make work, because swap and migration just make it much more
> complicated. Anyhow, future work]
>
> >
> >> is it an acceptable way to do the below in do_swap_page?
> >> if (!folio_test_anon(folio))
> >>        folio_add_new_anon_rmap()
> >> else
> >>        folio_add_anon_rmap_ptes()
> >
> > I am curious to know the answer as well.
>
>
> Yes, the end code should likely be something like:
>
> /* ksm created a completely new copy */
> if (unlikely(folio != swapcache && swapcache)) {
>         folio_add_new_anon_rmap(folio, vma, vmf->address);
>         folio_add_lru_vma(folio, vma);
> } else if (folio_test_anon(folio)) {
>         folio_add_anon_rmap_ptes(rmap_flags)
> } else {
>         folio_add_new_anon_rmap(rmap_flags)
> }
>
> Maybe we want to avoid teaching all existing folio_add_new_anon_rmap()
> callers about a new flag, and just have a new
> folio_add_new_shared_anon_rmap() instead. TBD.

There is more than one caller needed to perform that dance around
folio_test_anon() then decide which function to call. It would be nice
to have a wrapper function folio_add_new_shared_anon_rmap() to
abstract this behavior.


>
> >
> > BTW, that test might have a race as well. By the time the task got
> > !anon result, this result might get changed by another task. We need
> > to make sure in the caller context this race can't happen. Otherwise
> > we can't do the above safely.
> Again, folio lock. Observe the folio_lock_or_retry() call that covers
> our existing folio_add_new_anon_rmap/folio_add_anon_rmap_pte calls.

Ack. Thanks for the explanation.

Chris