Messages in this thread | | | Subject | Re: [PATCH] mm: Limit boost_watermark on small zones. | From | Henry Willard <> | Date | Mon, 4 May 2020 17:27:55 -0700 |
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On 5/4/20 1:36 PM, Andrew Morton wrote: > On Mon, 4 May 2020 13:44:09 +0100 Mel Gorman <mgorman@techsingularity.net> 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 <henry.willard@oracle.com> 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 <mgorman@techsingularity.net> > 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.
Thanks, Henry
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