linux-kernel - Re: [PATCH] mm: ensure alloc_flags in slow path are initialized

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Date:   Mon, 23 Jan 2017 15:56:38 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: ensure alloc_flags in slow path are initialized

On Mon, 23 Jan 2017 13:16:12 +0100 Arnd Bergmann <arnd@...db.de> wrote:

> The __alloc_pages_slowpath() has gotten rather complex and gcc
> is no longer able to follow the gotos and prove that the
> alloc_flags variable is initialized at the time it is used:
> 
> mm/page_alloc.c: In function '__alloc_pages_slowpath':
> mm/page_alloc.c:3565:15: error: 'alloc_flags' may be used uninitialized in this function [-Werror=maybe-uninitialized]
> 
> To be honest, I can't figure that out either, maybe it is or
> maybe not, but moving the existing initialization up a little
> higher looks safe and makes it obvious to both me and gcc that
> the initialization comes before the first use.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3591,6 +3591,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  				(__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
>  		gfp_mask &= ~__GFP_ATOMIC;
>  
> +	/*
> +	 * The fast path uses conservative alloc_flags to succeed only until
> +	 * kswapd needs to be woken up, and to avoid the cost of setting up
> +	 * alloc_flags precisely. So we do that now.
> +	 */
> +	alloc_flags = gfp_to_alloc_flags(gfp_mask);
> +
>  retry_cpuset:
>  	compaction_retries = 0;
>  	no_progress_loops = 0;
> @@ -3607,14 +3614,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  	if (!ac->preferred_zoneref->zone)
>  		goto nopage;
>  
> -
> -	/*
> -	 * The fast path uses conservative alloc_flags to succeed only until
> -	 * kswapd needs to be woken up, and to avoid the cost of setting up
> -	 * alloc_flags precisely. So we do that now.
> -	 */
> -	alloc_flags = gfp_to_alloc_flags(gfp_mask);
> -
>  	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
>  		wake_all_kswapds(order, ac);

hm.  But we later do

	if (gfp_pfmemalloc_allowed(gfp_mask))
		alloc_flags = ALLOC_NO_WATERMARKS;

	...
	if (read_mems_allowed_retry(cpuset_mems_cookie))
		goto retry_cpuset;

so with your patch there's a path where we can rerun everything with
alloc_flags == ALLOC_NO_WATERMARKS.  That's changed behaviour.

When I saw the test robot warning I did this, which I think preserves
behaviour?

--- a/mm/page_alloc.c~mm-consolidate-gfp_nofail-checks-in-the-allocator-slowpath-fix
+++ a/mm/page_alloc.c
@@ -3577,6 +3577,14 @@ retry_cpuset:
 	no_progress_loops = 0;
 	compact_priority = DEF_COMPACT_PRIORITY;
 	cpuset_mems_cookie = read_mems_allowed_begin();
+
+	/*
+	 * The fast path uses conservative alloc_flags to succeed only until
+	 * kswapd needs to be woken up, and to avoid the cost of setting up
+	 * alloc_flags precisely. So we do that now.
+	 */
+	alloc_flags = gfp_to_alloc_flags(gfp_mask);
+
 	/*
 	 * We need to recalculate the starting point for the zonelist iterator
 	 * because we might have used different nodemask in the fast path, or
@@ -3588,14 +3596,6 @@ retry_cpuset:
 	if (!ac->preferred_zoneref->zone)
 		goto nopage;
 
-
-	/*
-	 * The fast path uses conservative alloc_flags to succeed only until
-	 * kswapd needs to be woken up, and to avoid the cost of setting up
-	 * alloc_flags precisely. So we do that now.
-	 */
-	alloc_flags = gfp_to_alloc_flags(gfp_mask);
-
 	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
 		wake_all_kswapds(order, ac);
 
_