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

From: Vlastimil Babka
Date: Tue Jan 24 2017 - 04:19:27 EST


On 01/24/2017 12:56 AM, Andrew Morton wrote:
> On Mon, 23 Jan 2017 13:16:12 +0100 Arnd Bergmann <arnd@xxxxxxxx> 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.

Right.

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

Yes, that's cleaner. Thanks.

> --- 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);
>
> _
>