Re: [PATCH] mm: thp: update split_queue_len correctly

[Shakeel Butt <shakeelb@google.com>]

On Tue, Nov 23, 2021 at 10:51 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Tue, Nov 23, 2021 at 9:47 AM Shakeel Butt <shakeelb@google.com> wrote:
> >
> > The deferred THPs are split on memory pressure through shrinker
> > callback and splitting of THP during reclaim can fail for several
> > reasons like unable to lock the THP, under writeback or unexpected
> > number of pins on the THP. Such pages are put back on the deferred split
> > list for consideration later. However kernel does not update the
> > deferred queue size on putting back the pages whose split was failed.
> > This patch fixes that.
> >
> > Fixes: 364c1eebe453 ("mm: thp: extract split_queue_* into a struct")
> > Signed-off-by: Shakeel Butt <shakeelb@google.com>
> > ---
> >  mm/huge_memory.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index e5483347291c..4fff9584815b 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2809,7 +2809,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >         unsigned long flags;
> >         LIST_HEAD(list), *pos, *next;
> >         struct page *page;
> > -       int split = 0;
> > +       unsigned long split = 0, num = 0;
> >
> >  #ifdef CONFIG_MEMCG
> >         if (sc->memcg)
> > @@ -2823,6 +2823,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >                 page = compound_head(page);
> >                 if (get_page_unless_zero(page)) {
> >                         list_move(page_deferred_list(page), &list);
> > +                       num++;
>
> Thanks for catching this. But I don't think "num" is needed, isn't the
> below code good enough?

Yes you are right. I will send the next version. I will at least
change the type of split to unsigned long.

>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e5483347291c..1fbd8299db0c 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2847,6 +2847,7 @@ static unsigned long deferred_split_scan(struct
> shrinker *shrink,
>
>         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
>         list_splice_tail(&list, &ds_queue->split_queue);
> +       ds_queue->split_queue_len -= split;
>         spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
>
>         /*
>
> >                 } else {
> >                         /* We lost race with put_compound_page() */
> >                         list_del_init(page_deferred_list(page));
> > @@ -2847,6 +2848,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >
> >         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >         list_splice_tail(&list, &ds_queue->split_queue);
> > +       ds_queue->split_queue_len += (num - split);
> >         spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> >
> >         /*
> > --
> > 2.34.0.rc2.393.gf8c9666880-goog
> >