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

[Yin, Fengwei]

On 3/18/2024 10:16 AM, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>
> > Hi Yin Fengwei,
> >
> > On 15/03/2024 11:12, David Hildenbrand wrote:
> > > 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?
I don't think READ_ONCE() can replace the lock.

> > > 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/
> >
> > Do you have any recollection of why you added the data_race() markup?
>
> Per my understanding, this is used to mark that the code accesses
> folio->_deferred_list without lock intentionally, while
> folio->_deferred_list may be changed in parallel.  IIUC, this is what
> data_race() is used for.  Or, my understanding is wrong?
Yes. This is my understanding also.


Regards
Yin, Fengwei

> --
> Best Regards,
> Huang, Ying