From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197])
	by kanga.kvack.org (Postfix) with ESMTP id 428146B0012
	for <linux-mm@kvack.org>; Wed, 18 Apr 2018 14:49:25 -0400 (EDT)
Received: by mail-pf0-f197.google.com with SMTP id x184so1398346pfd.14
        for <linux-mm@kvack.org>; Wed, 18 Apr 2018 11:49:25 -0700 (PDT)
Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65])
        by mx.google.com with SMTPS id z5sor439939pgo.64.2018.04.18.11.49.24
        for <linux-mm@kvack.org>
        (Google Transport Security);
        Wed, 18 Apr 2018 11:49:24 -0700 (PDT)
Date: Wed, 18 Apr 2018 11:49:22 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
Subject: Re: [PATCH] SLUB: Do not fallback to mininum order if __GFP_NORETRY
 is set
In-Reply-To: <alpine.LRH.2.02.1804181102490.13213@file01.intranet.prod.int.rdu2.redhat.com>
Message-ID: <alpine.DEB.2.21.1804181147100.227784@chino.kir.corp.google.com>
References: <alpine.DEB.2.20.1804180944180.1062@nuc-kabylake> <alpine.LRH.2.02.1804181102490.13213@file01.intranet.prod.int.rdu2.redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: Mikulas Patocka <mpatocka@redhat.com>
Cc: Christopher Lameter <cl@linux.com>, Vlastimil Babka <vbabka@suse.cz>, Mike Snitzer <snitzer@redhat.com>, Matthew Wilcox <willy@infradead.org>, Pekka Enberg <penberg@kernel.org>, linux-mm@kvack.org, dm-devel@redhat.com, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org

On Wed, 18 Apr 2018, Mikulas Patocka 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@linux.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;
>  		/*
> 

I don't see the connection between the max order, which can be influenced 
by userspace with slub_min_objects, slub_min_order, etc, and specifying 
__GFP_NORETRY which means try to reclaim and free memory but don't loop.

If I force a slab cache to try a max order of 9 for hugepages as a best 
effort, why does __GFP_NORETRY suddenly mean I won't fallback to 
oo_order(s->min)?