[PATCH] mm, hugetlb: allow hugepage allocations to excessively reclaim

[Michal Hocko]

From: David Rientjes <rientjes@xxxxxxxxxx>

b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction
may not succeed") has chnaged the allocator to bail out from the
allocator early to prevent from a potentially excessive memory
reclaim. __GFP_RETRY_MAYFAIL is designed to retry the allocation,
reclaim and compaction loop as long as there is a reasonable chance to
make a forward progress. Neither COMPACT_SKIPPED nor COMPACT_DEFERRED
at the INIT_COMPACT_PRIORITY compaction attempt gives this feedback.

The most obvious affected subsystem is hugetlbfs which allocates huge
pages based on an admin request (or via admin configured overcommit).
I have done a simple test which tries to allocate half of the memory
for hugetlb pages while the memory is full of a clean page cache. This
is not an unusual situation because we try to cache as much of the
memory as possible and sysctl/sysfs interface to allocate huge pages is
there for flexibility to allocate hugetlb pages at any time.

System has 1GB of RAM and we are requesting 515MB worth of hugetlb pages
after the memory is prefilled by a clean page cache:
root@test1:~# cat hugetlb_test.sh

set -x
echo 0 > /proc/sys/vm/nr_hugepages
echo 3 > /proc/sys/vm/drop_caches
echo 1 > /proc/sys/vm/compact_memory
dd if=/mnt/data/file-1G of=/dev/null bs=$((4<<10))
TS=$(date +%s)
echo 256 > /proc/sys/vm/nr_hugepages
cat /proc/sys/vm/nr_hugepages

The results for 2 consecutive runs on clean 5.3
root@test1:~# sh hugetlb_test.sh
+ echo 0
+ echo 3
+ echo 1
+ dd if=/mnt/data/file-1G of=/dev/null bs=4096
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 21.0694 s, 51.0 MB/s
+ date +%s
+ TS=1569905284
+ echo 256
+ cat /proc/sys/vm/nr_hugepages
256
root@test1:~# sh hugetlb_test.sh
+ echo 0
+ echo 3
+ echo 1
+ dd if=/mnt/data/file-1G of=/dev/null bs=4096
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 21.7548 s, 49.4 MB/s
+ date +%s
+ TS=1569905311
+ echo 256
+ cat /proc/sys/vm/nr_hugepages
256

Now with b39d0ee2632d applied
root@test1:~# sh hugetlb_test.sh
+ echo 0
+ echo 3
+ echo 1
+ dd if=/mnt/data/file-1G of=/dev/null bs=4096
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 20.1815 s, 53.2 MB/s
+ date +%s
+ TS=1569905516
+ echo 256
+ cat /proc/sys/vm/nr_hugepages
11
root@test1:~# sh hugetlb_test.sh
+ echo 0
+ echo 3
+ echo 1
+ dd if=/mnt/data/file-1G of=/dev/null bs=4096
262144+0 records in
262144+0 records out
1073741824 bytes (1.1 GB) copied, 21.9485 s, 48.9 MB/s
+ date +%s
+ TS=1569905541
+ echo 256
+ cat /proc/sys/vm/nr_hugepages
12

The success rate went down by factor of 20!

Although hugetlb allocation requests might fail and it is reasonable to
expect them to under extremely fragmented memory or when the memory is
under a heavy pressure but the above situation is not that case.

Fix the regression by reverting back to the previous behavior for
__GFP_RETRY_MAYFAIL requests and disable the beail out heuristic for
those requests.

[mhocko@xxxxxxxx: reworded changelog]
Fixes: b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction may not succeed")
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---

Hi,
this has been posted by David as an RFC [1]. David doesn't seem to
appreciate the level of regression so I have largely rewritten the
changelog to be more explicit. I haven't changed the patch itself
so I have preserved his s-o-b.

I would also like to emphasise that I am not overly happy about the
patch. Vlastimil has posted [2] an alternative solution which looks
better but it is also slightly more complex. We can do that in a follow
up though so let's go with the simplest hack^Wsolution for now.

[1] http://lkml.kernel.org/r/alpine.DEB.2.21.1910021556270.187014@xxxxxxxxxxxxxxxxxxxxxxxxx
[2] http://lkml.kernel.org/r/20191001054343.GA15624@xxxxxxxxxxxxxx

 mm/page_alloc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 15c2050c629b..01aa46acee76 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4467,12 +4467,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		if (page)
 			goto got_pg;
 
-		 if (order >= pageblock_order && (gfp_mask & __GFP_IO)) {
+		 if (order >= pageblock_order && (gfp_mask & __GFP_IO) &&
+		     !(gfp_mask & __GFP_RETRY_MAYFAIL)) {
 			/*
 			 * If allocating entire pageblock(s) and compaction
 			 * failed because all zones are below low watermarks
 			 * or is prohibited because it recently failed at this
-			 * order, fail immediately.
+			 * order, fail immediately unless the allocator has
+			 * requested compaction and reclaim retry.
 			 *
 			 * Reclaim is
 			 *  - potentially very expensive because zones are far
-- 
2.20.1