linux-kernel - Re: [PATCH] SLUB: Do not fallback to mininum order if __GFP

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Date:   Thu, 19 Apr 2018 13:00:51 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Christopher Lameter <cl@...ux.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Pekka Enberg <penberg@...nel.org>, linux-mm@...ck.org,
        dm-devel@...hat.com, David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] SLUB: Do not fallback to mininum order if __GFP_NORETRY
 is set

On Wed 18-04-18 09:45:39, Cristopher Lameter wrote:
> Mikulas Patoka wants to ensure that no fallback to lower order happens. I
> think __GFP_NORETRY should work correctly in that case too and not fall
> back.

Overriding __GFP_NORETRY is just a bad idea. It will make the semantic
of the flag just more confusing. Note there are users who use
__GFP_NORETRY as a way to suppress heavy memory pressure and/or the OOM
killer. You do not want to change the semantic for them.

Besides that the changelog is less than optimal. What is the actual
problem? Why somebody doesn't want a fallback? Is there a configuration
that could prevent the same?

> Allocating at a smaller order is a retry operation and should not
> be attempted.
> 
> If the caller does not want retries then respect that.
> 
> GFP_NORETRY allows callers to ensure that only maximum order
> allocations are attempted.
> 
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> Index: linux/mm/slub.c
> ===================================================================
> --- linux.orig/mm/slub.c
> +++ linux/mm/slub.c
> @@ -1598,7 +1598,7 @@ static struct page *allocate_slab(struct
>  		alloc_gfp = (alloc_gfp | __GFP_NOMEMALLOC) & ~(__GFP_RECLAIM|__GFP_NOFAIL);
> 
>  	page = alloc_slab_page(s, alloc_gfp, node, oo);
> -	if (unlikely(!page)) {
> +	if (unlikely(!page) && !(flags & __GFP_NORETRY)) {
>  		oo = s->min;
>  		alloc_gfp = flags;
>  		/*

-- 
Michal Hocko
SUSE Labs