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Subject: Re: [PATCH] mm: Limit boost_watermark on small zones.
To:     Henry Willard <henry.willard@oracle.com>, akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <1588294148-6586-1-git-send-email-henry.willard@oracle.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <2c614ed5-ce72-b439-a6bd-0b24a041ac52@redhat.com>
Date:   Tue, 5 May 2020 10:30:49 +0200
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On 01.05.20 02:49, Henry Willard wrote:
> Commit 1c30844d2dfe ("mm: reclaim small amounts of memory when an external
> fragmentation event occurs") adds a boost_watermark() function which
> increases the min watermark in a zone by at least pageblock_nr_pages or
> the number of pages in a page block. On Arm64, with 64K pages and 512M
> huge pages, this is 8192 pages or 512M. It does this regardless of the
> number of managed pages managed in the zone or the likelihood of success.
> This can put the zone immediately under water in terms of allocating pages
> from the zone, and can cause a small machine to fail immediately due to
> OoM. Unlike set_recommended_min_free_kbytes(), which substantially
> increases min_free_kbytes and is tied to THP, boost_watermark() can be
> called even if THP is not active. The problem is most likely to appear
> on architectures such as Arm64 where pageblock_nr_pages is very large.
> 
> It is desirable to run the kdump capture kernel in as small a space as
> possible to avoid wasting memory. In some architectures, such as Arm64,
> there are restrictions on where the capture kernel can run, and therefore,
> the space available. A capture kernel running in 768M can fail due to OoM
> immediately after boost_watermark() sets the min in zone DMA32, where
> most of the memory is, to 512M. It fails even though there is over 500M of
> free memory. With boost_watermark() suppressed, the capture kernel can run
> successfully in 448M.
> 
> This patch limits boost_watermark() to boosting a zone's min watermark only
> when there are enough pages that the boost will produce positive results.
> In this case that is estimated to be four times as many pages as
> pageblock_nr_pages.
> 
> Signed-off-by: Henry Willard <henry.willard@oracle.com>

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb