From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751228AbVLCKDX (ORCPT ); Sat, 3 Dec 2005 05:03:23 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750905AbVLCKDX (ORCPT ); Sat, 3 Dec 2005 05:03:23 -0500 Received: from gold.veritas.com ([143.127.12.110]:32083 "EHLO gold.veritas.com") by vger.kernel.org with ESMTP id S1751228AbVLCKDW (ORCPT ); Sat, 3 Dec 2005 05:03:22 -0500 Date: Sat, 3 Dec 2005 10:03:04 +0000 (GMT) From: Hugh Dickins X-X-Sender: hugh@goblin.wat.veritas.com To: Blaisorblade cc: Andrew Morton , LKML , linux-mm@vger.kernel.org Subject: Re: [2.6.15-rc1+ regression] do_file_page bug introduced in recent rework In-Reply-To: <200512020111.56671.blaisorblade@yahoo.it> Message-ID: References: <200512020111.56671.blaisorblade@yahoo.it> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-OriginalArrivalTime: 03 Dec 2005 10:03:07.0140 (UTC) FILETIME=[C225DC40:01C5F7F0] Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2 Dec 2005, Blaisorblade wrote: > I recently found a bug introduced in your commit > 65500d234e74fc4e8f18e1a429bc24e51e75de4a, i.e. between 2.6.14 and 2.6.15-rc1, > about do_file_page changes wrt remap_file_pages and MAP_POPULATE. > > Quoting from the changelog (which is wrong): > > do_file_page's fallback to do_no_page dates from a time when we were > testing > pte_file by using it wherever possible: currently it's peculiar to > nonlinear > vmas, so just check that. BUG_ON if not? Better not, it's probably page > table corruption, so just show the pte: hmm, there's a pte_ERROR macro, > let's > use that for do_wp_page's invalid pfn too. > > This is false: > > do_mmap_pgoff: > if (flags & MAP_POPULATE) { > up_write(&mm->mmap_sem); > sys_remap_file_pages(addr, len, 0, > pgoff, flags & MAP_NONBLOCK); > down_write(&mm->mmap_sem); > } > > So, with MAP_POPULATE|MAP_NONBLOCK passed, you can get a linear PAGE_FILE pte > in a !VM_NONLINEAR vma. > > That PTE is very useless since it doesn't add any information, I know that, so > avoiding that possible installation is a possible fix, but for now it's > simpler to change the test in do_file_page(). Btw, in fact I discovered this > bug while I was implementing this optimization (working again on > remap_file_pages() patches of this summer). > > Indeed, the condition to test (and to possibly BUG_ON/pte_ERROR) is that > ->populate must exist for the sys_remap_file_pages call to work. I'm puzzled. Both filemap_populate and shmem_populate now test VM_NONLINEAR before calling install_file_pte. Hugh