On Sat, May 14, 2011 at 6:31 AM, Andrew Barry [off-list ref] wrote:

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.

Is there a reason that this loop is set up this way for 0 order allocations? I
applied the below patch, and the problem corrects itself. Does anyone have any
thoughts on the patch, or on a better way to address this situation?

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.

It's amazing.
I think it's _very_ rare but it's possible if test program killed by
oom has only lots of anonymous pages and allocation tasks try to
allocate order-0 page with GFP_NOFS.

When the [in]active lists are empty suddenly(But I am not sure how
come the situation happens.) and we are reclaiming order-0 page,
compaction and __alloc_pages_direct_reclaim doesn't work. compaction
doesn't work as it's order-0 page reclaiming.  In case of
__alloc_pages_direct_reclaim, it would work only if we have lru pages
in [in]active list. But unfortunately we don't have any pages in lru
list.
So, last resort is following codes in do_try_to_free_pages.

        /* top priority shrink_zones still had more to do? don't OOM, then */
        if (scanning_global_lru(sc) && !all_unreclaimable(zonelist, sc))
                return 1;

But it has a problem, too. all_unreclaimable checks zone->all_unreclaimable.
zone->all_unreclaimable is set by below condition.

zone->pages_scanned < zone_reclaimable_pages(zone) * 6

If lru list is completely empty, shrink_zone doesn't work so
zone->pages_scanned would be zero. But as we know, zone_page_state
isn't exact by per_cpu_pageset. So it might be positive value. After
all, zone_reclaimable always return true. It means kswapd never set
zone->all_unreclaimable.  So last resort become nop.

In this case, current allocation doesn't have a chance to call
get_page_from_freelist as Andrew Barry said.

Does it make sense?
If it is, how about this?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ebc7faa..4f64355 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -2105,6 +2105,7 @@ 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,

@@ -2112,7 +2113,6 @@ 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,

-- 
Kind regards,
Minchan Kim

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>