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Date: Mon, 4 May 2020 13:44:09 +0100 From: Mel Gorman <mgorman@...hsingularity.net> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Henry Willard <henry.willard@...cle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] mm: Limit boost_watermark on small zones. 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@...cle.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. > > > > Let's Cc Mel. > Seems reasonable. Acked-by: Mel Gorman <mgorman@...hsingularity.net> -- Mel Gorman SUSE Labs
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