From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Subject: [rfc][patch] mm: dirty page accounting hole Date: Tue, 12 Aug 2008 15:58:39 +1000 MIME-Version: 1.0 Content-Type: Multipart/Mixed; boundary="Boundary-00=_QaSoIF/hcJSK0Ib" Message-Id: <200808121558.40130.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org Return-Path: To: linux-mm@kvack.org Cc: "Dickins, Hugh" , "Zijlstra, Peter" List-ID: --Boundary-00=_QaSoIF/hcJSK0Ib Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Disposition: inline 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), 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... Anyway, here's a patch to fix this first particular issue... --Boundary-00=_QaSoIF/hcJSK0Ib Content-Type: text/x-diff; charset="us-ascii"; name="mm-dirty-account-fix.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="mm-dirty-account-fix.patch" Index: linux-2.6/mm/rmap.c =================================================================== --- linux-2.6.orig/mm/rmap.c +++ linux-2.6/mm/rmap.c @@ -481,7 +481,7 @@ static int page_mkclean_file(struct addr spin_lock(&mapping->i_mmap_lock); vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) { - if (vma->vm_flags & VM_SHARED) + if (vma->vm_flags & VM_MAYSHARE) ret += page_mkclean_one(page, vma); } spin_unlock(&mapping->i_mmap_lock); --Boundary-00=_QaSoIF/hcJSK0Ib-- -- 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