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

[Michal Hocko]

On Wed 13-09-17 14:19:19, Vlastimil Babka wrote:
> On 09/13/2017 02:14 PM, Michal Hocko wrote:
> >>>> Do you think that the changelog should be more clear about this?
> >>>
> >>> It certainly wouldn't hurt :)
> >>
> >> So what do you think about the following wording:
> > 
> > Ups, wrong patch
> > 
> > 
> > From 8639496a834b4a7c24972ec23b17e50f0d6a304c Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > Date: Mon, 14 Aug 2017 10:46:12 +0200
> > Subject: [PATCH 1/2] mm, memory_hotplug: do not fail offlining too early
> > 
> > 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>
> 
> Yeah, that's better, thanks.
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks. I will give it a day and repost the series. If somebody still
have some concerns please speak up.

-- 
Michal Hocko
SUSE Labs