[PATCH 1/2] mm, memory_hotplug: do not fail offlining too early

[Michal Hocko]

Memory offlining can fail just too eagerly under a heavy memory pressure.

[ 5410.336792] page:ffffea22a646bd00 count:255 mapcount:252 mapping:ffff88ff926c9f38 index:0x3
[ 5410.336809] flags: 0x9855fe40010048(uptodate|active|mappedtodisk)
[ 5410.336811] page dumped because: isolation failed
[ 5410.336813] page->mem_cgroup:ffff8801cd662000
[ 5420.655030] memory offlining [mem 0x18b580000000-0x18b5ffffffff] failed

Isolation has failed here because the page is not on LRU. Most probably
because it was on the pcp LRU cache or it has been removed from the LRU
already but it hasn't been freed yet. In both cases the page doesn't look
non-migrable so retrying more makes sense.

__offline_pages seems rather cluttered when it comes to the retry
logic. We have 5 retries at maximum and a timeout. We could argue
whether the timeout makes sense but failing just because of a race when
somebody isoltes a page from LRU or puts it on a pcp LRU lists is just
wrong. It only takes it to race with a process which unmaps some pages
and remove them from the LRU list and we can fail the whole offline
because of something that is a temporary condition and actually not
harmful for the offline.

Please note that unmovable pages should be already excluded during
start_isolate_page_range. We could argue that has_unmovable_pages is
racy and MIGRATE_MOVABLE check doesn't provide any hard guarantee either
but kernel zones (aka < ZONE_MOVABLE) will very likely detect unmovable
pages in most cases and movable zone shouldn't contain unmovable pages
at all. Some of those pages might be pinned but not for ever because
that would be a bug on its own. In any case the context is still
interruptible and so the userspace can easily bail out when the
operation takes too long. This is certainly better behavior than a
hardcoded retry loop which is racy.

Fix this by removing the max retry count and only rely on the timeout
resp. interruption by a signal from the userspace. Also retry rather
than fail when check_pages_isolated sees some !free pages because those
could be a result of the race as well.

Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

-- 
Michal Hocko
SUSE Labs