From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id AA0DE6B0033 for ; Wed, 11 Jan 2017 02:06:23 -0500 (EST) Received: by mail-pg0-f69.google.com with SMTP id 5so1278036423pgj.6 for ; Tue, 10 Jan 2017 23:06:23 -0800 (PST) Received: from mail-pf0-x231.google.com (mail-pf0-x231.google.com. [2607:f8b0:400e:c00::231]) by mx.google.com with ESMTPS id k25si4851808pfg.178.2017.01.10.23.06.22 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 10 Jan 2017 23:06:22 -0800 (PST) Received: by mail-pf0-x231.google.com with SMTP id y143so39229747pfb.0 for ; Tue, 10 Jan 2017 23:06:22 -0800 (PST) Date: Tue, 10 Jan 2017 23:06:10 -0800 (PST) From: Hugh Dickins 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> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: 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 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 -- 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