[PATCH] Prevent unending loop in __alloc_pages_slowpath

[Minchan Kim]

From: Andrew Barry <abarry@xxxxxxxx>

I believe I found a problem in __alloc_pages_slowpath, which allows a process to
get stuck endlessly looping, even when lots of memory is available.

Running an I/O and memory intensive stress-test I see a 0-order page allocation
with __GFP_IO and __GFP_WAIT, running on a system with very little free memory.
Right about the same time that the stress-test gets killed by the OOM-killer,
the utility trying to allocate memory gets stuck in __alloc_pages_slowpath even
though most of the systems memory was freed by the oom-kill of the stress-test.

The utility ends up looping from the rebalance label down through the
wait_iff_congested continiously. Because order=0, __alloc_pages_direct_compact
skips the call to get_page_from_freelist. Because all of the reclaimable memory
on the system has already been reclaimed, __alloc_pages_direct_reclaim skips the
call to get_page_from_freelist. Since there is no __GFP_FS flag, the block with
__alloc_pages_may_oom is skipped. The loop hits the wait_iff_congested, then
jumps back to rebalance without ever trying to get_page_from_freelist. This loop
repeats infinitely.

The test case is pretty pathological. Running a mix of I/O stress-tests that do
a lot of fork() and consume all of the system memory, I can pretty reliably hit
this on 600 nodes, in about 12 hours. 32GB/node.

Signed-off-by: Andrew Barry <abarry@xxxxxxxx>
Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
---
 mm/page_alloc.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3f8bce2..e78b324 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2064,6 +2064,7 @@ restart:
 		first_zones_zonelist(zonelist, high_zoneidx, NULL,
 					&preferred_zone);
 
+rebalance:
 	/* This is the last chance, in general, before the goto nopage. */
 	page = get_page_from_freelist(gfp_mask, nodemask, order, zonelist,
 			high_zoneidx, alloc_flags & ~ALLOC_NO_WATERMARKS,
@@ -2071,7 +2072,6 @@ restart:
 	if (page)
 		goto got_pg;
 
-rebalance:
 	/* Allocate without watermarks if the context allows */
 	if (alloc_flags & ALLOC_NO_WATERMARKS) {
 		page = __alloc_pages_high_priority(gfp_mask, order,
-- 
1.7.1


> 
> > -Andrew
> >
> >
> >
> >
> >
> >
> 
> 
> 
> -- 
> Kind regards,
> Minchan Kim

-- 
Kind regards,
Minchan Kim
--
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