Re: [PATCH 4/5] mm: fix check_move_unevictable_pages() on THP

[Yang Shi]

On Sun, Aug 30, 2020 at 2:08 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> check_move_unevictable_pages() is used in making unevictable shmem pages
> evictable: by shmem_unlock_mapping(), drm_gem_check_release_pagevec() and
> i915/gem check_release_pagevec().  Those may pass down subpages of a huge
> page, when /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force".
>
> That does not crash or warn at present, but the accounting of vmstats
> unevictable_pgs_scanned and unevictable_pgs_rescued is inconsistent:
> scanned being incremented on each subpage, rescued only on the head
> (since tails already appear evictable once the head has been updated).
>
> 5.8 commit 5d91f31faf8e ("mm: swap: fix vmstats for huge page") has
> established that vm_events in general (and unevictable_pgs_rescued in
> particular) should count every subpage: so follow that precedent here.
>
> Do this in such a way that if mem_cgroup_page_lruvec() is made stricter
> (to check page->mem_cgroup is always set), no problem: skip the tails
> before calling it, and add thp_nr_pages() to vmstats on the head.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Acked-by: Yang Shi <shy828301@xxxxxxxxx>

> ---
> Nothing here worth going to stable, since it's just a testing config
> that is fixed, whose event numbers are not very important; but this
> will be needed before Alex Shi's warning, and might as well go in now.
>
> The callers of check_move_unevictable_pages() could be optimized,
> to skip over tails: but Matthew Wilcox has other changes in flight
> there, so let's skip the optimization for now.
>
>  mm/vmscan.c |   10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> --- 5.9-rc2/mm/vmscan.c 2020-08-16 17:32:50.721507348 -0700
> +++ linux/mm/vmscan.c   2020-08-28 17:47:10.595580876 -0700
> @@ -4260,8 +4260,14 @@ void check_move_unevictable_pages(struct
>         for (i = 0; i < pvec->nr; i++) {
>                 struct page *page = pvec->pages[i];
>                 struct pglist_data *pagepgdat = page_pgdat(page);
> +               int nr_pages;
> +
> +               if (PageTransTail(page))
> +                       continue;
> +
> +               nr_pages = thp_nr_pages(page);
> +               pgscanned += nr_pages;
>
> -               pgscanned++;
>                 if (pagepgdat != pgdat) {
>                         if (pgdat)
>                                 spin_unlock_irq(&pgdat->lru_lock);
> @@ -4280,7 +4286,7 @@ void check_move_unevictable_pages(struct
>                         ClearPageUnevictable(page);
>                         del_page_from_lru_list(page, lruvec, LRU_UNEVICTABLE);
>                         add_page_to_lru_list(page, lruvec, lru);
> -                       pgrescued++;
> +                       pgrescued += nr_pages;
>                 }
>         }
>
>