linux-kernel - Re: [PATCH 11/12] mm: accurate pageout congestion wait

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 5 Apr 2007 16:17:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	root@...gramming.kicks-ass.net
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, a.p.zijlstra@...llo.nl,
	nikita@...sterfs.com
Subject: Re: [PATCH 11/12] mm: accurate pageout congestion wait

On Thu, 05 Apr 2007 19:42:20 +0200
root@...gramming.kicks-ass.net wrote:

> Only do the congestion wait when we actually encountered congestion.

The name congestion_wait() was accurate back in 2002, but it isn't accurate
any more, and you got misled.  It does not only wait for a queue to become
uncongested.

See clear_bdi_congested()'s callers.  As long as the queue is in an
uncongested state, we deliver wakeups to congestion_wait() blockers on
every IO completion.  As I said before, it is so that the MM's polling
operations poll at a higher frequency when the IO system is working faster.
(It is also to synchronise with end_page_writeback()'s feeding of clean
pages to us via rotate_reclaimable_page()).

Page reclaim can get into trouble without any request queue having entered
a congested state.  For example, think about a machine which has a single
disk, and the operator has increased that disk's request queue size to
100,000.  With your patch all the VM's throttling would be bypassed and we
go into a busy loop and declare OOM instantly.

There are probably other situations in which page reclaim gets into trouble
without a request queue being congested.

Minor point: bdi_congested() can be arbitrarily expensive - for DM stackups
it is roughly proportional to the number of subdevices in the device.  We
need to be careful about how frequently we call it.

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/