On Thu, 30 Apr 2020 17:49:08 -0700 Henry Willard <henry.will...@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. > Let's Cc Mel. > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -2400,6 +2400,14 @@ static inline void boost_watermark(struct zone *zone) > > if (!watermark_boost_factor) > return; > + /* > + * Don't bother in zones that are unlikely to produce results. > + * On small machines, including kdump capture kernels running > + * in a small area, boosting the watermark can cause an out of > + * memory situation immediately. > + */ > + if ((pageblock_nr_pages * 4) > zone_managed_pages(zone)) > + return; > > max_boost = mult_frac(zone->_watermark[WMARK_HIGH], > watermark_boost_factor, 10000); > -- > 1.8.3.1