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

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

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