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@xxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---
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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