On Mon, May 04, 2020 at 01:36:04PM -0700, Andrew Morton wrote: > On Mon, 4 May 2020 13:44:09 +0100 Mel Gorman <[email protected]> > 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 > > > <[email protected]> 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 <[email protected]> > > 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?
There is no harm in marking it stable. Clearly it does not happen very often but it's not impossible. 32-bit x86 is a lot less common now which would previously have been vulnerable to triggering this easily. ppc64 has a larger base page size but typically only has one zone. arm64 is likely the most vulnerable, particularly when CMA is configured with a small movable zone. -- Mel Gorman SUSE Labs
