From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx188.postini.com [74.125.245.188]) by kanga.kvack.org (Postfix) with SMTP id CCB106B006E for ; Mon, 1 Oct 2012 12:22:31 -0400 (EDT) Date: Mon, 1 Oct 2012 18:22:26 +0200 From: Michal Hocko Subject: Re: [PATCH] hugetlb: do not use vma_hugecache_offset for vma_prio_tree_foreach Message-ID: <20121001162226.GA24860@dhcp22.suse.cz> References: <1344866141-27906-1-git-send-email-mhocko@suse.cz> <20120926205617.GA2667@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120926205617.GA2667@cmpxchg.org> Sender: owner-linux-mm@kvack.org List-ID: To: Johannes Weiner Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hillf Danton , Mel Gorman , KAMEZAWA Hiroyuki , Andrea Arcangeli , David Rientjes On Wed 26-09-12 16:56:17, Johannes Weiner wrote: > On Mon, Aug 13, 2012 at 03:55:41PM +0200, Michal Hocko wrote: > > 0c176d5 (mm: hugetlb: fix pgoff computation when unmapping page > > from vma) fixed pgoff calculation but it has replaced it by > > vma_hugecache_offset which is not approapriate for offsets used for > > vma_prio_tree_foreach because that one expects index in page units > > rather than in huge_page_shift. > > Using vma_hugecache_offset is not incorrect because the pgoff will fit > > into the same vmas but it is confusing so the standard PAGE_SHIFT based > > index calculation is used instead. > > I do think it's incorrect. The resulting index may not be too big, > but it can be too small: assume hpage size of 2M and the address to > unmap to be 0x200000. This is regular page index 512 and hpage index > 1. If you have a VMA that maps the file only starting at the second > huge page, that VMAs vm_pgoff will be 512 but you ask for offset 1 and > miss it even though it does map the page of interest. hugetlb_cow() > will try to unmap, miss the vma, and retry the cow until the > allocation succeeds or the skipped vma(s) go away. > > Unless I missed something, this should not be deferred as a cleanup. You are right and I have totally missed this because I focused on the other boundary too much :/ This vma_hugecache_offset is really confusing. Andrew has already updated the changelog so we will not get even more confusion into the Linus tree. Thanks for spotting this Johannes! -- Michal Hocko SUSE Labs -- 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