From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id 2C9F66007E3 for ; Wed, 2 Dec 2009 15:13:47 -0500 (EST) Date: Wed, 2 Dec 2009 20:13:39 +0000 (GMT) From: Hugh Dickins Subject: Re: [PATCH] hugetlb: Acquire the i_mmap_lock before walking the prio_tree to unmap a page In-Reply-To: <20091202141930.GF1457@csn.ul.ie> Message-ID: References: <20091202141930.GF1457@csn.ul.ie> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org To: Mel Gorman Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org List-ID: 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. Hugh > --- > mm/hugetlb.c | 9 ++++++++- > 1 files changed, 8 insertions(+), 1 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index a952cb8..5adc284 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -1906,6 +1906,12 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma, > + (vma->vm_pgoff >> PAGE_SHIFT); > mapping = (struct address_space *)page_private(page); > > + /* > + * Take the mapping lock for the duration of the table walk. As > + * this mapping should be shared between all the VMAs, > + * __unmap_hugepage_range() is called as the lock is already held > + */ > + spin_lock(&mapping->i_mmap_lock); > vma_prio_tree_foreach(iter_vma, &iter, &mapping->i_mmap, pgoff, pgoff) { > /* Do not unmap the current VMA */ > if (iter_vma == vma) > @@ -1919,10 +1925,11 @@ static int unmap_ref_private(struct mm_struct *mm, struct vm_area_struct *vma, > * from the time of fork. This would look like data corruption > */ > if (!is_vma_resv_set(iter_vma, HPAGE_RESV_OWNER)) > - unmap_hugepage_range(iter_vma, > + __unmap_hugepage_range(iter_vma, > address, address + huge_page_size(h), > page); > } > + spin_unlock(&mapping->i_mmap_lock); > > return 1; > } -- 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