Re: [PATCH 6/6] mm: discard __GFP_ATOMIC

[Vlastimil Babka <vbabka@suse.cz>]

On 11/29/22 16:17, Mel Gorman wrote:
> From: NeilBrown <neilb@suse.de>
> 
> __GFP_ATOMIC serves little purpose.  Its main effect is to set
> ALLOC_HARDER which adds a few little boosts to increase the chance of an
> allocation succeeding, one of which is to lower the water-mark at which it
> will succeed.
> 
> It is *always* paired with __GFP_HIGH which sets ALLOC_HIGH which also
> adjusts this watermark.  It is probable that other users of __GFP_HIGH
> should benefit from the other little bonuses that __GFP_ATOMIC gets.
> 
> __GFP_ATOMIC also gives a warning if used with __GFP_DIRECT_RECLAIM.
> There is little point to this.  We already get a might_sleep() warning if
> __GFP_DIRECT_RECLAIM is set.
> 
> __GFP_ATOMIC allows the "watermark_boost" to be side-stepped.  It is
> probable that testing ALLOC_HARDER is a better fit here.
> 
> __GFP_ATOMIC is used by tegra-smmu.c to check if the allocation might
> sleep.  This should test __GFP_DIRECT_RECLAIM instead.
> 
> This patch:
>  - removes __GFP_ATOMIC
>  - allows __GFP_HIGH allocations to ignore watermark boosting as well
>    as GFP_ATOMIC requests.
>  - makes other adjustments as suggested by the above.
> 
> The net result is not change to GFP_ATOMIC allocations.  Other
> allocations that use __GFP_HIGH will benefit from a few different extra
> privileges.  This affects:
>   xen, dm, md, ntfs3
>   the vermillion frame buffer
>   hibernation
>   ksm
>   swap
> all of which likely produce more benefit than cost if these selected
> allocation are more likely to succeed quickly.
> 
> [mgorman: Minor adjustments to rework on top of a series]
> Link: https://lkml.kernel.org/r/163712397076.13692.4727608274002939094@noble.neil.brown.name
> Signed-off-by: NeilBrown <neilb@suse.de>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Just a nit below.


> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4081,13 +4081,14 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
>  	if (__zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
>  					free_pages))
>  		return true;
> +
>  	/*
> -	 * Ignore watermark boosting for GFP_ATOMIC order-0 allocations
> +	 * Ignore watermark boosting for GFP_HIGH order-0 allocations

There's no GFP_HIGH. We could either keep GFP_ATOMIC here if we want to talk
about the high-level flag combo, or __GFP_HIGH if about the low-level
detail. We're deep in the page allocator implementation so the latter would
be OK.

>  	 * when checking the min watermark. The min watermark is the
>  	 * point where boosting is ignored so that kswapd is woken up
>  	 * when below the low watermark.
>  	 */
> -	if (unlikely(!order && (gfp_mask & __GFP_ATOMIC) && z->watermark_boost
> +	if (unlikely(!order && (alloc_flags & ALLOC_MIN_RESERVE) && z->watermark_boost