From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759571AbdAKHGY (ORCPT ); Wed, 11 Jan 2017 02:06:24 -0500 Received: from mail-pf0-f172.google.com ([209.85.192.172]:36010 "EHLO mail-pf0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756774AbdAKHGX (ORCPT ); Wed, 11 Jan 2017 02:06:23 -0500 Date: Tue, 10 Jan 2017 23:06:10 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: "Kirill A. Shutemov" cc: Keno Fischer , linux-kernel@vger.kernel.org, linux-mm@kvack.org, torvalds@linux-foundation.org, gthelen@google.com, npiggin@gmail.com, w@1wt.eu, oleg@redhat.com, keescook@chromium.org, luto@kernel.org, mhocko@suse.com, rientjes@google.com, hughd@google.com Subject: Re: [PATCH] mm: Respect FOLL_FORCE/FOLL_COW for thp In-Reply-To: Message-ID: References: <20170105053658.GA36383@juliacomputing.com> <20170105150558.GE17319@node.shutemov.name> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 10 Jan 2017, Hugh Dickins wrote: > On Thu, 5 Jan 2017, Kirill A. Shutemov wrote: > > On Thu, Jan 05, 2017 at 12:36:58AM -0500, Keno Fischer wrote: > > > struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr, > > > pmd_t *pmd, int flags) > > > { > > > @@ -783,7 +793,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr, > > > > > > assert_spin_locked(pmd_lockptr(mm, pmd)); > > > > > > - if (flags & FOLL_WRITE && !pmd_write(*pmd)) > > > + if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags)) > > > return NULL; > > > > I don't think this part is needed: once we COW devmap PMD entry, we split > > it into PTE table, so IIUC we never get here with PMD. > > Hi Kirill, > > Would you mind double-checking that? You certainly know devmap > better than me, but I feel safer with Keno's original as above. > > I can see that fs/dax.c dax_iomap_pmd_fault() does > > /* Fall back to PTEs if we're going to COW */ > if (write && !(vma->vm_flags & VM_SHARED)) > goto fallback; > > But isn't there a case of O_RDWR fd, VM_SHARED PROT_READ mmap, and > FOLL_FORCE write to it, which does not COW (but relies on FOLL_COW)? And now I think I'm wrong, but please double-check even so: I think that case gets ruled out by the !is_cow_mapping(vm_flags) check in mm/gup.c, where we used to have a WARN_ON_ONCE() for a while. Hugh