From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Pavel Tatashin <pasha.tatashin@soleen.com>,
Oscar Salvador <osalvador@suse.de>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH 9/9] mm, page_alloc: optionally disable pcplists during page isolation
Date: Tue, 6 Oct 2020 10:40:23 +0200 [thread overview]
Message-ID: <a35c1f92-b56a-e6c4-9920-33f99850eb76@redhat.com> (raw)
In-Reply-To: <20201006083418.GB29020@dhcp22.suse.cz>
On 06.10.20 10:34, Michal Hocko wrote:
> On Tue 22-09-20 16:37:12, Vlastimil Babka wrote:
>> Page isolation can race with process freeing pages to pcplists in a way that
>> a page from isolated pageblock can end up on pcplist. This can be fixed by
>> repeated draining of pcplists, as done by patch "mm/memory_hotplug: drain
>> per-cpu pages again during memory offline" in [1].
>>
>> David and Michal would prefer that this race was closed in a way that callers
>> of page isolation who need stronger guarantees don't need to repeatedly drain.
>> David suggested disabling pcplists usage completely during page isolation,
>> instead of repeatedly draining them.
>>
>> To achieve this without adding special cases in alloc/free fastpath, we can use
>> the same approach as boot pagesets - when pcp->high is 0, any pcplist addition
>> will be immediately flushed.
>>
>> The race can thus be closed by setting pcp->high to 0 and draining pcplists
>> once, before calling start_isolate_page_range(). The draining will serialize
>> after processes that already disabled interrupts and read the old value of
>> pcp->high in free_unref_page_commit(), and processes that have not yet disabled
>> interrupts, will observe pcp->high == 0 when they are rescheduled, and skip
>> pcplists. This guarantees no stray pages on pcplists in zones where isolation
>> happens.
>>
>> This patch thus adds zone_pcplist_disable() and zone_pcplist_enable() functions
>> that page isolation users can call before start_isolate_page_range() and after
>> unisolating (or offlining) the isolated pages. A new zone->pcplist_disabled
>> atomic variable makes sure we disable only pcplists once and don't enable
>> them prematurely in case there are multiple users in parallel.
>>
>> We however have to avoid external updates to high and batch by taking
>> pcp_batch_high_lock. To allow multiple isolations in parallel, change this lock
>> from mutex to rwsem.
>
> The overall idea makes sense. I just suspect you are over overcomplicating
> the implementation a bit. Is there any reason that we cannot start with
> a really dumb implementation first. The only user of this functionality
> is the memory offlining and that is already strongly synchronized
> (mem_hotplug_begin) so a lot of trickery can be dropped here. Should we
> find a new user later on we can make the implementation finer grained
> but now it will not serve any purpose. So can we simply update pcp->high
> and drain all pcp in the given zone and wait for all remote pcp draining
> in zone_pcplist_enable and updte revert all that in zone_pcplist_enable.
> We can stick to the existing pcp_batch_high_lock.
>
> What do you think?
>
My two cents, we might want to make use of this in some cases of
alloc_contig_range() soon ("try hard mode"). So I'd love to see a
synchronized mechanism. However, that can be factored out into a
separate patch, so this patch gets significantly simpler.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2020-10-06 8:40 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-22 14:37 [PATCH 0/9] disable pcplists during memory offline Vlastimil Babka
2020-09-22 14:37 ` [PATCH 1/9] mm, page_alloc: clean up pageset high and batch update Vlastimil Babka
2020-09-25 10:18 ` David Hildenbrand
2020-10-05 12:03 ` Michal Hocko
2020-09-22 14:37 ` [PATCH 2/9] mm, page_alloc: calculate pageset high and batch once per zone Vlastimil Babka
2020-10-05 12:52 ` Michal Hocko
2020-10-06 22:04 ` Vlastimil Babka
2020-09-22 14:37 ` [PATCH 3/9] mm, page_alloc: remove setup_pageset() Vlastimil Babka
2020-09-25 10:19 ` David Hildenbrand
2020-10-05 12:59 ` Michal Hocko
2020-10-06 22:11 ` Vlastimil Babka
2020-09-22 14:37 ` [PATCH 4/9] mm, page_alloc: simplify pageset_update() Vlastimil Babka
2020-09-25 10:23 ` David Hildenbrand
2020-10-05 13:20 ` Michal Hocko
2020-09-22 14:37 ` [PATCH 5/9] mm, page_alloc: make per_cpu_pageset accessible only after init Vlastimil Babka
2020-09-25 10:25 ` David Hildenbrand
2020-10-05 13:24 ` Michal Hocko
2020-10-06 22:28 ` Vlastimil Babka
2020-09-22 14:37 ` [PATCH 6/9] mm, page_alloc: cache pageset high and batch in struct zone Vlastimil Babka
2020-09-25 10:34 ` David Hildenbrand
2020-10-06 22:31 ` Vlastimil Babka
2020-10-05 13:28 ` Michal Hocko
2020-10-06 22:34 ` Vlastimil Babka
2020-09-22 14:37 ` [PATCH 7/9] mm, page_alloc: move draining pcplists to page isolation users Vlastimil Babka
2020-09-25 10:39 ` David Hildenbrand
2020-10-05 13:57 ` Michal Hocko
2020-09-22 14:37 ` [PATCH 8/9] mm, page_alloc: drain all pcplists during memory offline Vlastimil Babka
2020-09-25 10:46 ` David Hildenbrand
2020-10-05 14:03 ` Michal Hocko
2020-09-22 14:37 ` [PATCH 9/9] mm, page_alloc: optionally disable pcplists during page isolation Vlastimil Babka
2020-09-25 10:53 ` David Hildenbrand
2020-09-25 10:54 ` David Hildenbrand
2020-09-25 11:10 ` Vlastimil Babka
2020-10-01 8:47 ` David Hildenbrand
2020-10-05 14:05 ` Michal Hocko
2020-10-05 14:22 ` Vlastimil Babka
2020-10-05 16:56 ` Michal Hocko
2020-10-06 8:34 ` Michal Hocko
2020-10-06 8:40 ` David Hildenbrand [this message]
2020-10-06 10:05 ` Michal Hocko
2020-09-22 17:15 ` [PATCH 0/9] disable pcplists during memory offline David Hildenbrand
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=a35c1f92-b56a-e6c4-9920-33f99850eb76@redhat.com \
--to=david@redhat.com \
--cc=iamjoonsoo.kim@lge.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@suse.com \
--cc=osalvador@suse.de \
--cc=pasha.tatashin@soleen.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).