Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[David Hildenbrand]

On 15.03.24 11:49, Ryan Roberts wrote:
> On 15/03/2024 10:43, David Hildenbrand wrote:
> > On 11.03.24 16:00, Ryan Roberts wrote:
> > > Now that swap supports storing all mTHP sizes, avoid splitting large
> > > folios before swap-out. This benefits performance of the swap-out path
> > > by eliding split_folio_to_list(), which is expensive, and also sets us
> > > up for swapping in large folios in a future series.
> > >
> > > If the folio is partially mapped, we continue to split it since we want
> > > to avoid the extra IO overhead and storage of writing out pages
> > > uneccessarily.
> > >
> > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> > > ---
> > >    mm/vmscan.c | 9 +++++----
> > >    1 file changed, 5 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index cf7d4cf47f1a..0ebec99e04c6 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > > @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head
> > > *folio_list,
> > >                        if (!can_split_folio(folio, NULL))
> > >                            goto activate_locked;
> > >                        /*
> > > -                     * Split folios without a PMD map right
> > > -                     * away. Chances are some or all of the
> > > -                     * tail pages can be freed without IO.
> > > +                     * Split partially mapped folios map
> > > +                     * right away. Chances are some or all
> > > +                     * of the tail pages can be freed
> > > +                     * without IO.
> > >                         */
> > > -                    if (!folio_entire_mapcount(folio) &&
> > > +                    if (!list_empty(&folio->_deferred_list) &&
> > >                            split_folio_to_list(folio,
> > >                                    folio_list))
> > >                            goto activate_locked;
> >
> > Not sure if we might have to annotate that with data_race().
>
> I asked that exact question to Matthew in another context bt didn't get a
> response. There are examples of checking if the deferred list is empty with and
> without data_race() in the code base. But list_empty() is implemented like this:
>
> static inline int list_empty(const struct list_head *head)
> {
> 	return READ_ONCE(head->next) == head;
> }
>
> So I assumed the READ_ONCE() makes everything safe without a lock? Perhaps not
> sufficient for KCSAN?

Yeah, there is only one use of data_race with that list.

It was added in f3ebdf042df4 ("THP: avoid lock when check whether THP is 
in deferred list").

Looks like that was added right in v1 of that change [1], so my best 
guess is that it is not actually required.

If not required, likely we should just cleanup the single user.

[1] 
https://lore.kernel.org/linux-mm/20230417075643.3287513-2-fengwei.yin@xxxxxxxxx/

--
Cheers,

David / dhildenb