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

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Date:   Tue, 23 Nov 2021 09:59:11 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     David Hildenbrand <david@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: thp: update split_queue_len correctly

On Tue, Nov 23, 2021 at 9:47 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.

I forgot to add the user visible impact.

"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."