linux-kernel - Re: [RFC 1/2] mm: additional page lock debugging

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Date:	Tue, 31 Dec 2013 11:42:04 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Kirill A. Shutemov" <kirill@...temov.name>,
	akpm@...ux-foundation.org, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC 1/2] mm: additional page lock debugging

On 12/31/2013 11:26 AM, Peter Zijlstra wrote:
> On Mon, Dec 30, 2013 at 10:22:02PM -0500, Sasha Levin wrote:
>
>> I really want to use lockdep here, but I'm not really sure how to handle locks which live
>> for a rather long while instead of being locked and unlocked in the same function like
>> most of the rest of the kernel. (Cc Ingo, PeterZ).
>
> Uh what? Lockdep doesn't care about which function locks and unlocks a
> particular lock. Nor does it care how long its held for.

Sorry, I messed up trying to explain that.

There are several places in the code which lock a large amount of pages, something like:

	for (i = 0; i < (1 << order); i++)
		lock_page(&pages[i]);


This triggers two problems:

  - lockdep complains about deadlock since we try to lock another page while one is already
locked. I can clear that by allowing page locks to nest within each other, but that seems
wrong and we'll miss actual deadlock cases.

  - We may leave back to userspace with pages still locked. This is valid behaviour but lockdep
doesn't like that.


Thanks,
Sasha

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