linux-kernel - Re: [PATCH v2] mm: thp: update split_queue

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Date:   Tue, 23 Nov 2021 12:00:19 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zi Yan <ziy@...dia.com>, Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm: thp: update split_queue_len correctly

On Tue, Nov 23, 2021 at 11:09 AM Shakeel Butt <shakeelb@...gle.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.
>
> Without this patch the split_queue_len can underflow. Shrinker will
> always get that there are some THPs to split even if there are not and
> waste some cpu to scan the empty list.
>
> Fixes: 364c1eebe453 ("mm: thp: extract split_queue_* into a struct")
> Signed-off-by: Shakeel Butt <shakeelb@...gle.com>
> ---
> Changes since v1:
> - updated commit message
> - incorporated Yang Shi's suggestion

Reviewed-by: Yang Shi <shy828301@...il.com>

>
>  mm/huge_memory.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e5483347291c..d393028681e2 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;
>
>  #ifdef CONFIG_MEMCG
>         if (sc->memcg)
> @@ -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);
>
>         /*
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>