linux-kernel - Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0

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Date:   Fri, 8 Oct 2021 13:47:00 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Vasily Averin <vvs@...tuozzo.com>, Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...nvz.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0

On 10/8/21 11:24, Vasily Averin wrote:
> __alloc_pages_bulk can call __count_zid_vm_events and zone_statistics
> with nr_account = 0.

But that's not a bug, right? Just an effective no-op that's not commonly
happening, so is it worth the check?

> Fixes: 3e23060b2d0b ("mm/page_alloc: batch the accounting updates in the bulk allocator")
> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
> ---
>  mm/page_alloc.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 602819a232e5..e67113452ee8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5364,9 +5364,10 @@ unsigned long __alloc_pages_bulk(gfp_t gfp, int preferred_nid,
>  	}
>  
>  	local_unlock_irqrestore(&pagesets.lock, flags);
> -
> -	__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
> -	zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
> +	if (nr_account) {
> +		__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
> +		zone_statistics(ac.preferred_zoneref->zone, zone, nr_account);
> +	}
>  	if (objcg)
>  		memcg_bulk_post_charge_hook(objcg, nr_pre_charge - nr_account);
>  
>