Re: [PATCH] mm: Limit boost_watermark on small zones.

[Andrew Morton]

On Thu, 30 Apr 2020 17:49:08 -0700 Henry Willard <henry.willard@xxxxxxxxxx> 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