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Subject[PATCH] mm: Do not stall in synchronous compaction for THP allocations
Occasionally during large file copies to slow storage, there are still
reports of user-visible stalls when THP is enabled. Reports on this
have been intermittent and not reliable to reproduce locally but;

Andy Isaacson reported a problem copying to VFAT on SD Card
https://lkml.org/lkml/2011/11/7/2

In this case, it was stuck in munmap for betwen 20 and 60
seconds in compaction. It is also possible that khugepaged
was holding mmap_sem on this process if CONFIG_NUMA was set.

Johannes Weiner reported stalls on USB
https://lkml.org/lkml/2011/7/25/378

In this case, there is no stack trace but it looks like the
same problem. The USB stick may have been using NTFS as a
filesystem based on other work done related to writing back
to USB around the same time.

Internally in SUSE, I received a bug report related to stalls in firefox
when using Java and Flash heavily while copying from NFS
to VFAT on USB. It has not been confirmed to be the same problem
but if it looks like a duck and quacks like a duck.....

In the past, commit [11bc82d6: mm: compaction: Use async migration for
__GFP_NO_KSWAPD and enforce no writeback] forced that sync compaction
would never be used for THP allocations. This was reverted in commit
[c6a140bf: mm/compaction: reverse the change that forbade sync
migraton with __GFP_NO_KSWAPD] on the grounds that it was uncertain
it was beneficial.

While user-visible stalls do not happen for me when writing to USB,
I setup a test running postmark while short-lived processes created
anonymous mapping. The objective was to exercise the paths that
allocate transparent huge pages. I then logged when processes were
stalled for more than 1 second, recorded a stack strace and did some
analysis to aggregate unique "stall events" which revealed

Time stalled in this event: 47369 ms
Event count: 20
usemem sleep_on_page 3690 ms
usemem sleep_on_page 2148 ms
usemem sleep_on_page 1534 ms
usemem sleep_on_page 1518 ms
usemem sleep_on_page 1225 ms
usemem sleep_on_page 2205 ms
usemem sleep_on_page 2399 ms
usemem sleep_on_page 2398 ms
usemem sleep_on_page 3760 ms
usemem sleep_on_page 1861 ms
usemem sleep_on_page 2948 ms
usemem sleep_on_page 1515 ms
usemem sleep_on_page 1386 ms
usemem sleep_on_page 1882 ms
usemem sleep_on_page 1850 ms
usemem sleep_on_page 3715 ms
usemem sleep_on_page 3716 ms
usemem sleep_on_page 4846 ms
usemem sleep_on_page 1306 ms
usemem sleep_on_page 1467 ms
[<ffffffff810ef30c>] wait_on_page_bit+0x6c/0x80
[<ffffffff8113de9f>] unmap_and_move+0x1bf/0x360
[<ffffffff8113e0e2>] migrate_pages+0xa2/0x1b0
[<ffffffff81134273>] compact_zone+0x1f3/0x2f0
[<ffffffff811345d8>] compact_zone_order+0xa8/0xf0
[<ffffffff811346ff>] try_to_compact_pages+0xdf/0x110
[<ffffffff810f773a>] __alloc_pages_direct_compact+0xda/0x1a0
[<ffffffff810f7d5d>] __alloc_pages_slowpath+0x55d/0x7a0
[<ffffffff810f8151>] __alloc_pages_nodemask+0x1b1/0x1c0
[<ffffffff811331db>] alloc_pages_vma+0x9b/0x160
[<ffffffff81142bb0>] do_huge_pmd_anonymous_page+0x160/0x270
[<ffffffff814410a7>] do_page_fault+0x207/0x4c0
[<ffffffff8143dde5>] page_fault+0x25/0x30

The stall times are approximate at best but the estimates represent 25%
of the worst stalls and even if the estimates are off by a factor of
10, it's severe.

This patch once again prevents sync migration for transparent
hugepage allocations as it is preferable to fail a THP allocation
than stall. It was suggested that __GFP_NORETRY be used instead of
__GFP_NO_KSWAPD. This would look less like a special case but would
still cause compaction to run at least once with sync compaction.

If accepted, this is a -stable candidate.

Reported-by: Andy Isaacson <adi@hexapodia.org>
Reported-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/page_alloc.c | 8 +++++++-
1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 963c5de..cddc2d0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2213,7 +2213,13 @@ rebalance:
sync_migration);
if (page)
goto got_pg;
- sync_migration = true;
+
+ /*
+ * Do not use sync migration for transparent hugepage allocations as
+ * it could stall writing back pages which is far worse than simply
+ * failing to promote a page.
+ */
+ sync_migration = !(gfp_mask & __GFP_NO_KSWAPD);

/* Try direct reclaim and then allocating */
page = __alloc_pages_direct_reclaim(gfp_mask, order,

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