linux-mm.kvack.org archive mirror
 help / color / mirror / Atom feed
From: Oscar Salvador <osalvador@suse.de>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Michal Hocko <mhocko@kernel.org>,
	Pavel Tatashin <pasha.tatashin@soleen.com>,
	David Hildenbrand <david@redhat.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH v2 4/7] mm, page_alloc: simplify pageset_update()
Date: Thu, 22 Oct 2020 14:39:58 +0200	[thread overview]
Message-ID: <20201022123958.GB26121@linux> (raw)
In-Reply-To: <20201008114201.18824-5-vbabka@suse.cz>

On Thu, Oct 08, 2020 at 01:41:58PM +0200, Vlastimil Babka wrote:
> pageset_update() attempts to update pcplist's high and batch values in a way
> that readers don't observe batch > high. It uses smp_wmb() to order the updates
> in a way to achieve this. However, without proper pairing read barriers in
> readers this guarantee doesn't hold, and there are no such barriers in
> e.g. free_unref_page_commit().
> 
> Commit 88e8ac11d2ea ("mm, page_alloc: fix core hung in free_pcppages_bulk()")
> already showed this is problematic, and solved this by ultimately only trusing
> pcp->count of the current cpu with interrupts disabled.
> 
> The update dance with unpaired write barriers thus makes no sense. Replace
> them with plain WRITE_ONCE to prevent store tearing, and document that the
> values can change asynchronously and should not be trusted for correctness.
> 
> All current readers appear to be OK after 88e8ac11d2ea. Convert them to
> READ_ONCE to prevent unnecessary read tearing, but mainly to alert anybody
> making future changes to the code that special care is needed.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

Yeah, I never got my head around those smp_wmb()

Reviewed-by: Oscar Salvador <osalvador@suse.de>

-- 
Oscar Salvador
SUSE L3


  reply	other threads:[~2020-10-22 12:40 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-08 11:41 [PATCH v2 0/7] disable pcplists during memory offline Vlastimil Babka
2020-10-08 11:41 ` [PATCH v2 1/7] mm, page_alloc: clean up pageset high and batch update Vlastimil Babka
2020-10-25 14:17   ` Mike Rapoport
2020-10-08 11:41 ` [PATCH v2 2/7] mm, page_alloc: calculate pageset high and batch once per zone Vlastimil Babka
2020-10-08 11:41 ` [PATCH v2 3/7] mm, page_alloc: remove setup_pageset() Vlastimil Babka
2020-10-08 12:23   ` Michal Hocko
2020-10-08 12:56     ` Vlastimil Babka
2020-10-08 13:03       ` Michal Hocko
2020-10-22 12:34   ` Oscar Salvador
2020-10-08 11:41 ` [PATCH v2 4/7] mm, page_alloc: simplify pageset_update() Vlastimil Babka
2020-10-22 12:39   ` Oscar Salvador [this message]
2020-10-08 11:41 ` [PATCH v2 5/7] mm, page_alloc: cache pageset high and batch in struct zone Vlastimil Babka
2020-10-08 12:31   ` Michal Hocko
2020-10-08 17:55     ` Vlastimil Babka
2020-10-22 12:42       ` Oscar Salvador
2020-10-08 11:42 ` [PATCH v2 6/7] mm, page_alloc: move draining pcplists to page isolation users Vlastimil Babka
2020-10-22 12:44   ` Oscar Salvador
2020-10-08 11:42 ` [PATCH v2 7/7] mm, page_alloc: disable pcplists during memory offline Vlastimil Babka
2020-10-08 12:45   ` Michal Hocko
2020-10-08 17:46     ` Vlastimil Babka
2020-10-22 12:52   ` Oscar Salvador

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=20201022123958.GB26121@linux \
    --to=osalvador@suse.de \
    --cc=david@redhat.com \
    --cc=iamjoonsoo.kim@lge.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@kernel.org \
    --cc=mhocko@suse.com \
    --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).