Re: [PATCH mm-unstable v3] mm/page_alloc: keep track of free highatomic

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Oct 28, 2024 at 12:26:53PM -0600, Yu Zhao wrote:
> OOM kills due to vastly overestimated free highatomic reserves were
> observed:
> 
>   ... invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0 ...
>   Node 0 Normal free:1482936kB boost:0kB min:410416kB low:739404kB high:1068392kB reserved_highatomic:1073152KB ...
>   Node 0 Normal: 1292*4kB (ME) 1920*8kB (E) 383*16kB (UE) 220*32kB (ME) 340*64kB (E) 2155*128kB (UE) 3243*256kB (UE) 615*512kB (U) 1*1024kB (M) 0*2048kB 0*4096kB = 1477408kB
> 
> The second line above shows that the OOM kill was due to the following
> condition:
> 
>   free (1482936kB) - reserved_highatomic (1073152kB) = 409784KB < min (410416kB)
> 
> And the third line shows there were no free pages in any
> MIGRATE_HIGHATOMIC pageblocks, which otherwise would show up as type
> 'H'. Therefore __zone_watermark_unusable_free() underestimated the
> usable free memory by over 1GB, which resulted in the unnecessary OOM
> kill above.
> 
> The comments in __zone_watermark_unusable_free() warns about the
> potential risk, i.e.,
> 
>   If the caller does not have rights to reserves below the min
>   watermark then subtract the high-atomic reserves. This will
>   over-estimate the size of the atomic reserve but it avoids a search.
> 
> However, it is possible to keep track of free pages in reserved
> highatomic pageblocks with a new per-zone counter nr_free_highatomic
> protected by the zone lock, to avoid a search when calculating the
> usable free memory. And the cost would be minimal, i.e., simple
> arithmetics in the highatomic alloc/free/move paths.
> 
> Note that since nr_free_highatomic can be relatively small, using a
> per-cpu counter might cause too much drift and defeat its purpose,
> in addition to the extra memory overhead.
> 
> Reported-by: Link Lin <linkl@google.com>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> Acked-by: David Rientjes <rientjes@google.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

> @@ -642,6 +644,9 @@ static inline void account_freepages(struct zone *zone, int nr_pages,
>  
>  	if (is_migrate_cma(migratetype))
>  		__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
> +
> +	if (is_migrate_highatomic(migratetype))
> +		WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic + nr_pages);

Minor nit, the page can only be of one migratetype, so `else if' would
be better.