Re: [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

[Vlastimil Babka <vbabka@suse.cz>]

On 10/26/24 05:36, 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

It's only possible to track this reliably since the "mm: page_alloc:
freelist migratetype hygiene" patchset was merged, which explains why
nr_reserved_highatomic was used until now, even if it's imprecise. But I
agree it should be a good solution to the problem now.

> usable free memory. And the cost would be minimal, i.e., simple
> arithmetics in the highatomic alloc/free/move paths.
> 
> Reported-by: Link Lin <linkl@google.com>
> Signed-off-by: Yu Zhao <yuzhao@google.com>

However, implementation wise that patchset also centralized everything to
account_freepages() which currently handles NR_FREE_PAGES and
NR_FREE_CMA_PAGES, and your patch differs from that approach. Can't you
simply add a NR_FREE_HIGHATOMIC update there instead of creating
account_highatomic_freepages() which is hooked to 3 different places? And
for consistency indeed add a zone_stat_item counter instead a struct zone
field? Then it becomes visible in e.g. /proc/zoneinfo and /proc/vmstat too.
Bonus points for printing it in the oom/alloc failure reports as well
(wherever we print NR_FREE_CMA_PAGES).

Thanks!
Vlastimil

> ---
>  include/linux/mmzone.h |  1 +
>  mm/page_alloc.c        | 23 ++++++++++++++++++++---
>  2 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 2e8c4307c728..5e8f567753bd 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -825,6 +825,7 @@ struct zone {
>  	unsigned long watermark_boost;
>  
>  	unsigned long nr_reserved_highatomic;
> +	unsigned long nr_free_highatomic;
>  
>  	/*
>  	 * We don't know if the memory that we're going to allocate will be
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c81762c49b00..43ecc7d75a2a 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -644,6 +644,18 @@ static inline void account_freepages(struct zone *zone, int nr_pages,
>  		__mod_zone_page_state(zone, NR_FREE_CMA_PAGES, nr_pages);
>  }
>  
> +static void account_highatomic_freepages(struct zone *zone, unsigned int order,
> +					 int old_mt, int new_mt)
> +{
> +	int delta = 1 << order;
> +
> +	if (is_migrate_highatomic(old_mt))
> +		WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic - delta);
> +
> +	if (is_migrate_highatomic(new_mt))
> +		WRITE_ONCE(zone->nr_free_highatomic, zone->nr_free_highatomic + delta);
> +}
> +
>  /* Used for pages not on another list */
>  static inline void __add_to_free_list(struct page *page, struct zone *zone,
>  				      unsigned int order, int migratetype,
> @@ -660,6 +672,8 @@ static inline void __add_to_free_list(struct page *page, struct zone *zone,
>  	else
>  		list_add(&page->buddy_list, &area->free_list[migratetype]);
>  	area->nr_free++;
> +
> +	account_highatomic_freepages(zone, order, -1, migratetype);
>  }
>  
>  /*
> @@ -681,6 +695,8 @@ static inline void move_to_free_list(struct page *page, struct zone *zone,
>  
>  	account_freepages(zone, -(1 << order), old_mt);
>  	account_freepages(zone, 1 << order, new_mt);
> +
> +	account_highatomic_freepages(zone, order, old_mt, new_mt);
>  }
>  
>  static inline void __del_page_from_free_list(struct page *page, struct zone *zone,
> @@ -698,6 +714,8 @@ static inline void __del_page_from_free_list(struct page *page, struct zone *zon
>  	__ClearPageBuddy(page);
>  	set_page_private(page, 0);
>  	zone->free_area[order].nr_free--;
> +
> +	account_highatomic_freepages(zone, order, migratetype, -1);
>  }
>  
>  static inline void del_page_from_free_list(struct page *page, struct zone *zone,
> @@ -3119,11 +3137,10 @@ static inline long __zone_watermark_unusable_free(struct zone *z,
>  
>  	/*
>  	 * 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.
> +	 * watermark then subtract the free pages reserved for highatomic.
>  	 */
>  	if (likely(!(alloc_flags & ALLOC_RESERVES)))
> -		unusable_free += z->nr_reserved_highatomic;
> +		unusable_free += READ_ONCE(z->nr_free_highatomic);
>  
>  #ifdef CONFIG_CMA
>  	/* If allocation can't use CMA areas don't use free CMA pages */