linux-kernel - Re: Deadlocks with transparent huge pages and userspace fs daemons

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Date:	Tue, 14 Dec 2010 18:46:26 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Lin Feng Shen <shenlinf@...ibm.com>,
	Yuri L Volobuev <volobuev@...ibm.com>,
	Mel Gorman <mel@...ux.vnet.ibm.com>, dingc@...ibm.com,
	lnxninja <lnxninja@...ibm.com>
Subject: Re: Deadlocks with transparent huge pages and userspace fs daemons

Hello Dave and everyone,

On Wed, Nov 03, 2010 at 01:43:25PM -0700, Dave Hansen wrote:
> Hey Miklos,
> 
> When testing with a transparent huge page kernel:
> 
> 	http://git.kernel.org/gitweb.cgi?p=linux/kernel/git/andrea/aa.git;a=summary
> 
> some IBM testers ran into some deadlocks.  It appears that the
> khugepaged process is trying to migrate one of a filesystem daemon's
> pages while khugepaged holds the daemon's mmap_sem for write.

The allocation under mmap_sem write mode in khugepaged bug should be
fixed in current aa.git based on 37-rc5:

http://git.kernel.org/?p=linux/kernel/git/andrea/aa.git;a=shortlog
http://git.kernel.org/?p=linux/kernel/git/andrea/aa.git;a=commit;h=83e4d55d0014b3eeb982005d73f55ffcf2813504

Let me know how it goes, it's not very well tested yet (which is why I
didn't make a new submit yet).

I stick to my idea this is bug in userland and may trigger if your
daemon does mmap/munmap and the vma allocation under mmap_sem waits
for the I/O, but I don't want to show it with THP enabled, and this is
more scalable so it's definitely good idea and no downside whatsoever.

Thanks for the report,
Andrea
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