From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: Re: [rfc][patch] mm: dirty page accounting hole Date: Tue, 12 Aug 2008 17:06:26 +1000 References: <200808121558.40130.nickpiggin@yahoo.com.au> <1218523840.10800.129.camel@twins> In-Reply-To: <1218523840.10800.129.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200808121706.26686.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org Return-Path: To: Peter Zijlstra Cc: linux-mm@kvack.org, "Dickins, Hugh" List-ID: On Tuesday 12 August 2008 16:50, Peter Zijlstra wrote: > On Tue, 2008-08-12 at 15:58 +1000, Nick Piggin wrote: > > Hi, > > > > 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. > > > > Then clear_page_dirty_for_io is called and leaves that pte dirty > > and cleans the page. It never gets cleaned until munmap, so msync > > and writeout accounting are broken. > > > > I have a fix which just scans VM_SHARED to VM_MAYSHARE. The other > > way I tried is to clear the dirty and write bits and set the page > > dirty in mprotect. The problem with that for me is that I'm trying > > to rework the vm/fs layer so we never have to allocate data to > > write out dirty pages (using page_mkwrite and dirty accounting), > > Ooh, nice! Thanks for confirming. > > and so this still leaves me with a window where the vma flags are > > changed but before the pte is marked clean, in which time the page > > is still dirty but it may have its metadata freed because it > > doesn't look dirty. > > > > There are several other problems I've also run into, including a > > fundamentally indadequate page_mkwrite locking scheme, which was > > naturally ignored when I brought it up during reviewing those > > patches. I digress... > > Yes, I remember you bringing that up, and later too when you did those > fault patches. I assumed you were 'working' on it. Oh that wasn't aimed at you, sorry ;) It is our general... "process" of development. I mean, there is apparently some impending apocolypse due to our scarcity of review bandwidth, so ignoring any review is by definition insignificant. -- 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