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

[Chris Li]

Hi David and Barry,

On Mon, Jan 22, 2024 at 10:49 PM Barry Song <21cnbao@xxxxxxxxx> 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.

> 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.

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.

Chris.