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

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Date:   Wed, 18 Apr 2018 11:05:31 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Christopher Lameter <cl@...ux.com>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        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 Apr 2018, Christopher 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.
> 
> 
> 
> 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;
>  		/*

No, this would hit NULL pointer dereference if page is NULL and 
__GFP_NORETRY is set. You want this:

---
 mm/slub.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6/mm/slub.c
===================================================================
--- linux-2.6.orig/mm/slub.c	2018-04-17 20:58:23.000000000 +0200
+++ linux-2.6/mm/slub.c	2018-04-18 17:04:01.000000000 +0200
@@ -1599,6 +1599,8 @@ static struct page *allocate_slab(struct
 
 	page = alloc_slab_page(s, alloc_gfp, node, oo);
 	if (unlikely(!page)) {
+		if (flags & __GFP_NORETRY)
+			goto out;
 		oo = s->min;
 		alloc_gfp = flags;
 		/*