From mboxrd@z Thu Jan 1 00:00:00 1970 From: Trond Myklebust Subject: Re: NFS BUG_ON in nfs_do_writepage Date: Tue, 28 Apr 2009 07:45:17 -0400 Message-ID: <1240919117.7376.6.camel@heimdal.trondhjem.org> References: <20090412235010.c8e3475b.akpm@linux-foundation.org> <1239650202.16771.15.camel@heimdal.trondhjem.org> <5da0588e0904131506k5c58e8ddob9bf38f61da6302a@mail.gmail.com> <5da0588e0904131644g131dc816r61884e83bc4cd006@mail.gmail.com> <5da0588e0904240226j3454941y5f58c17a32a9a23d@mail.gmail.com> <1240671428.6112.1.camel@heimdal.trondhjem.org> <20090426064026.GD28555@wotan.suse.de> <1240755509.5055.34.camel@heimdal.trondhjem.org> <20090426151324.GB5588@wotan.suse.de> <1240768522.10548.33.camel@heimdal.trondhjem.org> <20090428042717.GA6304@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Rince , Andrew Morton , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-nfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Nick Piggin Return-path: In-Reply-To: <20090428042717.GA6304-B4tOwbsTzaBolqkO4TVVkw@public.gmane.org> Sender: linux-nfs-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-fsdevel.vger.kernel.org On Tue, 2009-04-28 at 06:27 +0200, Nick Piggin wrote: > On Sun, Apr 26, 2009 at 01:55:22PM -0400, Trond Myklebust wrote: > > On Sun, 2009-04-26 at 17:13 +0200, Nick Piggin wrote: > > > This doesn't seem to fix the race, though... on kernels with the > > > race still there, it will just open a window where you can have > > > a dirty pte but the page not written out. > > > > > > I don't understand. > > > > I'm just pointing out that the NFS client already calls > > __set_page_dirty_nobuffers() while holding the page lock inside the > > nfs_vm_page_mkwrite() call, so having the VM do it too in the call to > > set_page_dirty_balance() is actually redundant. IOW: as far as the NFS > > code is concerned, we can get rid of the ->set_page_dirty() callback in > > that situation. > > > > I couldn't find any other places in the VM code where we can have a > > dirty pte without also having called page_mkwrite() (and hence > > __set_page_dirty_nobuffers). As I said, adding a WARN_ON(!PageDirty()) > > in ->set_page_dirty() didn't ever trigger any cases where the > > set_page_dirty() was actually setting the dirty bit (except in the case > > where we race with page writeout in do_wp_page() and __do_fault()). > > > > That's why I believe disabling ->set_page_dirty() is safe here, and will > > in fact suffice to fix the page writeout race. > > Ah, no I don't think so because it opens another race where the > pte is dity but the page is marked clean. So how can that happen? AFAICS, when the pte is dirtied, we should get a page fault, which causes the page itself to be marked dirty by the nfs_vm_page_mkwrite() callback. When the page gets written out, the VM calls clear_page_dirty_for_io() which also causes the pte to be cleaned. At what point can you therefore have a situation where the pte is dirty without the page being marked as dirty too? -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html