From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with ESMTP id 068566007E3 for ; Wed, 2 Dec 2009 17:19:53 -0500 (EST) Date: Wed, 2 Dec 2009 22:19:47 +0000 From: Mel Gorman Subject: Re: [PATCH] hugetlb: Acquire the i_mmap_lock before walking the prio_tree to unmap a page Message-ID: <20091202221947.GB26702@csn.ul.ie> References: <20091202141930.GF1457@csn.ul.ie> <20091202221602.GA26702@csn.ul.ie> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20091202221602.GA26702@csn.ul.ie> Sender: owner-linux-mm@kvack.org To: Hugh Dickins Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: On Wed, Dec 02, 2009 at 10:16:02PM +0000, Mel Gorman wrote: > On Wed, Dec 02, 2009 at 08:13:39PM +0000, Hugh Dickins wrote: > > On Wed, 2 Dec 2009, Mel Gorman wrote: > > > > > When the owner of a mapping fails COW because a child process is holding a > > > reference and no pages are available, the children VMAs are walked and the > > > page is unmapped. The i_mmap_lock is taken for the unmapping of the page but > > > not the walking of the prio_tree. In theory, that tree could be changing > > > while the lock is released although in practice it is protected by the > > > hugetlb_instantiation_mutex. This patch takes the i_mmap_lock properly for > > > the duration of the prio_tree walk in case the hugetlb_instantiation_mutex > > > ever goes away. > > > > > > [hugh.dickins@tiscali.co.uk: Spotted the problem in the first place] > > > Signed-off-by: Mel Gorman > > > > The patch looks good - thanks for taking care of that, Mel. > > > > But the comment seems wrong to me: hugetlb_instantiation_mutex > > guards against concurrent hugetlb_fault()s; but the structure of > > the prio_tree shifts as vmas based on that inode are inserted into > > (mmap'ed) and removed from (munmap'ed) that tree (always while > > holding i_mmap_lock). I don't see hugetlb_instantiation_mutex > > giving us any protection against this at present. > > > > You're right of course. I'll report without that nonsense included. > Actually, shouldn't the mmap_sem be protecting against concurrent mmap and munmap altering the tree? The comment is still bogus of course. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org