From: Oscar Salvador <osalvador@xxxxxxx>
Subject: mm,page_alloc: bail out earlier on -ENOMEM in alloc_contig_migrate_range
Patch series "Make alloc_contig_range handle Hugetlb pages", v10.
alloc_contig_range lacks the ability to handle HugeTLB pages. This can be
problematic for some users, e.g: CMA and virtio-mem, where those users
will fail the call if alloc_contig_range ever sees a HugeTLB page, even
when those pages lay in ZONE_MOVABLE and are free. That problem can be
easily solved by replacing the page in the free hugepage pool.
In-use HugeTLB are no exception though, as those can be isolated and
migrated as any other LRU or Movable page.
This patchset aims for improving
alloc_contig_range->isolate_migratepages_block, so HugeTLB pages can be
recognized and handled.
Since we also need to start reporting errors down the chain (e.g: -ENOMEM
due to not be able to allocate a new hugetlb page),
isolate_migratepages_{range,block} interfaces need to change to start
reporting error codes instead of the pfn == 0 vs pfn != 0 scheme it is
using right now. From now on, isolate_migratepages_block will not return
the next pfn to be scanned anymore, but -EINTR, -ENOMEM or 0, so we the
next pfn to be scanned will be recorded in cc->migrate_pfn field (as it is
already done in isolate_migratepages_range()).
Below is an insight from David (thanks), where the problem can clearly be
seen:
"Start a VM with 4G. Hotplug 1G via virtio-mem and online it to
ZONE_MOVABLE. Allocate 512 huge pages.
[root@localhost ~]# cat /proc/meminfo
MemTotal: 5061512 kB
MemFree: 3319396 kB
MemAvailable: 3457144 kB
...
HugePages_Total: 512
HugePages_Free: 512
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
The huge pages get partially allocate from ZONE_MOVABLE. Try unplugging
1G via virtio-mem (remember, all ZONE_MOVABLE). Inside the guest:
[ 180.058992] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[ 180.060531] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[ 180.061972] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[ 180.063413] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[ 180.064838] alloc_contig_range: [1b8000, 1c0000) PFNs busy
[ 180.065848] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[ 180.066794] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[ 180.067738] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[ 180.068669] alloc_contig_range: [1bfc00, 1c0000) PFNs busy
[ 180.069598] alloc_contig_range: [1bfc00, 1c0000) PFNs busy"
And then with this patchset running:
"Same experiment with ZONE_MOVABLE:
a) Free huge pages: all memory can get unplugged again.
b) Allocated/populated but idle huge pages: all memory can get unplugged
again.
c) Allocated/populated but all 512 huge pages are read/written in a
loop: all memory can get unplugged again, but I get a single
[ 121.192345] alloc_contig_range: [180000, 188000) PFNs busy
Most probably because it happened to try migrating a huge page
while it was busy. As virtio-mem retries on ZONE_MOVABLE a couple of
times, it can deal with this temporary failure.
Last but not least, I did something extreme:
# cat /proc/meminfo
MemTotal: 5061568 kB
MemFree: 186560 kB
MemAvailable: 354524 kB
...
HugePages_Total: 2048
HugePages_Free: 2048
HugePages_Rsvd: 0
HugePages_Surp: 0
Triggering unplug would require to dissolve+alloc - which now fails
when trying to allocate an additional ~512 huge pages (1G).
As expected, I can properly see memory unplug not fully succeeding. +
I get a fairly continuous stream of
[ 226.611584] alloc_contig_range: [19f400, 19f800) PFNs busy
...
But more importantly, the hugepage count remains stable, as configured
by the admin (me):
HugePages_Total: 2048
HugePages_Free: 2048
HugePages_Rsvd: 0
HugePages_Surp: 0"
This patch (of 7):
Currently, __alloc_contig_migrate_range can generate -EINTR, -ENOMEM or
-EBUSY, and report them down the chain. The problem is that when
migrate_pages() reports -ENOMEM, we keep going till we exhaust all the
try-attempts (5 at the moment) instead of bailing out.
migrate_pages() bails out right away on -ENOMEM because it is considered a
fatal error. Do the same here instead of keep going and retrying. Note
that this is not fixing a real issue, just a cosmetic change. Although we
can save some cycles by backing off ealier
Link: https://lkml.kernel.org/r/20210419075413.1064-1-osalvador@xxxxxxx
Link: https://lkml.kernel.org/r/20210419075413.1064-2-osalvador@xxxxxxx
Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Acked-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_alloc.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
--- a/mm/page_alloc.c~mmpage_alloc-bail-out-earlier-on-enomem-in-alloc_contig_migrate_range
+++ a/mm/page_alloc.c
@@ -8696,7 +8696,7 @@ static int __alloc_contig_migrate_range(
}
tries = 0;
} else if (++tries == 5) {
- ret = ret < 0 ? ret : -EBUSY;
+ ret = -EBUSY;
break;
}
@@ -8706,6 +8706,13 @@ static int __alloc_contig_migrate_range(
ret = migrate_pages(&cc->migratepages, alloc_migration_target,
NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
+
+ /*
+ * On -ENOMEM, migrate_pages() bails out right away. It is pointless
+ * to retry again over this error, so do the same here.
+ */
+ if (ret == -ENOMEM)
+ break;
}
if (ret < 0) {
alloc_contig_dump_pages(&cc->migratepages);
_
