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Date:   Mon, 4 May 2020 17:27:55 -0700
From:   Henry Willard <>
To:     Andrew Morton <>,
        Mel Gorman <>
Subject: Re: [PATCH] mm: Limit boost_watermark on small zones.

On 5/4/20 1:36 PM, Andrew Morton wrote:
> On Mon, 4 May 2020 13:44:09 +0100 Mel Gorman <> wrote:
>> On Fri, May 01, 2020 at 03:57:29PM -0700, Andrew Morton wrote:
>>> On Thu, 30 Apr 2020 17:49:08 -0700 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.
>> ...
>> Acked-by: Mel Gorman <>
> Cool.  I wonder if we should backport this into -stable kernels?  "can
> cause a small machine to fail immediately" sounds serious, but
> 1c30844d2dfe is from December 2018.  Any thoughts?
It is a trivial patch, and we always like to have them in -stable 
kernels when possible. It was a serious problem for us, because we had a 
configuration where kdump on Arm always failed. However, other than 
kdump, the problem is probably relatively rare.


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