linux-kernel - Re: [PATCH] Memory management livelock

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]

Message-ID: <Pine.LNX.4.64.0810030729290.4875@hs20-bc2-1.build.redhat.com>
Date:	Fri, 3 Oct 2008 07:43:46 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: Re: [PATCH] Memory management livelock

On Thu, 2 Oct 2008, Andrew Morton wrote:

> On Fri, 3 Oct 2008 13:47:21 +1000 Nick Piggin <nickpiggin@...oo.com.au> wrote:
> 
> > > I expect there's no solution which avoids blocking the writers at some
> > > stage.
> > 
> > See my other email. Something roughly like this would do the trick
> > (hey, it actually boots and runs and does fix the problem too).
> 
> It needs exclusion to protect all those temp tags.  Is do_fsync()'s
> i_mutex sufficient?  It's qute unobvious (and unmaintainable?) that all
> the callers of this stuff are running under that lock.

That filemap_fdatawrite and filemap_fdatawait in fsync() aren't really 
called under i_mutex (see do_fsync).

So the possible solutions are:

1. Add jiffies when the page was diried and wroteback to struct page
+ no impact on locking and concurrency
- increases the structure by 8 bytes

2. Stop the writers when the starvation happens (what I did)
+ doesn't do any locking if the livelock doesn't happen
- locks writers when the livelock happens (I think it's not really serious 
--- because very few people complained about the livelock, very few people 
will see performance degradation from blocking the writers).

3. Add another bit to radix tree (what Nick did)
+ doesn't ever block writers
- unconditionally takes the lock on fsync path and serializates concurrent 
syncs/fsyncs. Probably low overhead too ... or I don't know, is there any 
possible situation when more processes execute sync() in parallel and user 
would see degradations if those syncs were serialized?

Mikulas
--
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/