Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0

From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	virtualization@lists.linux-foundation.org,
	Andrea Arcangeli <aarcange@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Juergen Gross <jgross@suse.com>,
	Pavel Tatashin <pavel.tatashin@microsoft.com>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	Anthony Yznaga <anthony.yznaga@oracle.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Oscar Salvador <osalvador@suse.de>,
	Pingfan Liu <kernelfans@gmail.com>, Qian Cai <cai@lca.pw>,
	Dan Williams <dan.j.williams@intel.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Wei Yang <richardw.yang@linux.intel.com>,
	Alexander Potapenko <glider@google.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Matthew Wilcox <willy@infradead.org>, Yu Zhao <yuzhao@google.com>,
	Minchan Kim <minchan@kernel.org>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>
Subject: Re: [PATCH RFC v3 6/9] mm: Allow to offline PageOffline() pages with a reference count of 0
Date: Thu, 24 Oct 2019 10:51:28 +0200	[thread overview]
Message-ID: <ba7164c9-b98e-0ce1-358e-8b0d45fe3f48@redhat.com> (raw)
In-Reply-To: <20191024084241.GV17610@dhcp22.suse.cz>

On 24.10.19 10:42, Michal Hocko wrote:
> On Wed 23-10-19 12:03:51, David Hildenbrand wrote:
>>> Do you see any downsides?
>>
>> The only downside I see is that we get more false negatives on
>> has_unmovable_pages(), eventually resulting in the offlining stage after
>> isolation to loop forever (as some PageOffline() pages are not movable
>> (especially, XEN balloon, HyperV balloon), there won't be progress).
>>
>> I somewhat don't like forcing everybody that uses PageOffline() (especially
>> all users of balloon compaction) to implement memory notifiers just to avoid
>> that. Maybe, we even want to use PageOffline() in the future in the core
>> (e.g., for memory holes instead of PG_reserved or similar).
> 
> There is only a handful of those and we need to deal with them anyway.
> If you do not want to enforce them to create their own notifiers then we
> can accomodate the hotplug code. __test_page_isolated_in_pageblock resp.

Yeah, I would prefer offlining code to be able to deal with that without 
notifier changes for all users.

> the call chain up can distinguish temporary and permanent failures
> (EAGAIN vs. EBUSY). The current state when we always return EBUSY and
> keep retrying for ever is not optimal at all, right? A referenced PageOffline

Very right!

> could be an example of EBUSY all other failures where we are effectively
> waiting for pages to get freed finaly would be EAGAIN.

We have to watch out for PageOffline() pages that are actually movable 
(balloon compaction). But that doesn't sound too hard.
> 
> It is a bit late in the process because a large portion of the work has
> been done already but this doesn't sound like something to lose sleep
> over.
> 

Right. I'll look into that to find out if this would work. And see if I 
can reproduce what I described at all (theoretical thoughts) :)

Again, thanks for looking into this Michal!

-- 

Thanks,

David / dhildenb