From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [rfc][patch] mm: dirty page accounting hole From: Peter Zijlstra In-Reply-To: References: <200808121558.40130.nickpiggin@yahoo.com.au> Content-Type: text/plain Date: Tue, 12 Aug 2008 13:30:56 +0200 Message-Id: <1218540656.10800.188.camel@twins> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Hugh Dickins Cc: Nick Piggin , linux-mm@kvack.org List-ID: On Tue, 2008-08-12 at 12:15 +0100, Hugh Dickins wrote: > On Tue, 12 Aug 2008, Nick Piggin wrote: > > > > I think I'm running into a hole in dirty page accounting... > > > > What seems to be happening is that a page gets written to via a > > VM_SHARED vma. We then set the pte dirty, then mark the page dirty. > > Next, mprotect changes the vma so it is no longer writeable so it > > is no longer VM_SHARED. The pte is still dirty. > > I don't think you've got that right yet. > > mprotect can of course change vma->vm_flags to take VM_WRITE off, > making vma no longer writeable; but it shouldn't be touching > VM_SHARED. And a quick check with debugger confirms that. > > It's precisely because of mprotect that page_mkclean_one tests > VM_SHARED not VM_WRITE. Changing that to VM_MAYSHARE, as in your > patch below, should make no difference to correctness; but would > potentially make its loop less efficient (it would also go off to > check MAP_SHARED, PROT_READ, fd readonly mappings unnecessarily). > > Perhaps there's somewhere else that clears VM_SHARED by mistake? > Or another path through mprotect which does so? I haven't checked > further, hoping this will jolt you into a different realization. You are right, I cannot find a path through mprotect that unsets VM_SHARED either. Something which I failed to validate this morning. -- 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