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SubjectRe: [PATCH] mm: ensure alloc_flags in slow path are initialized
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On 01/23/2017 01:16 PM, Arnd Bergmann 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,

Seems the report is correct and not false positive, in scenario when we goto
nopage before the assignment, and then goto retry because of __GFP_NOFAIL.

> 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.
>
> Fixes: 74eaa4a97e8e ("mm: consolidate GFP_NOFAIL checks in the allocator slowpath")

That's a non-stable -next commit ID for mmotm patch:
mm-consolidate-gfp_nofail-checks-in-the-allocator-slowpath.patch

The patch itself was OK, the problem only comes from integration with another
mmotm patch (also independently OK):
mm-page_alloc-fix-premature-oom-when-racing-with-cpuset-mems-update.patch

By their ordering in mmotm, it would work to treat this as a fix for the
GFP_NOFAIL patch, possibly merged into it.

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/page_alloc.c | 15 +++++++--------
> 1 file changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index cf641932c015..d9fa4564524f 100644
> --- 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);
>
>

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