From mboxrd@z Thu Jan 1 00:00:00 1970 In-reply-to: <20070307021518.3b1ff4a2.akpm@linux-foundation.org> (message from Andrew Morton on Wed, 7 Mar 2007 02:15:18 -0800) Subject: Re: [patch 1/8] fix race in clear_page_dirty_for_io() References: <20070307080949.290171170@szeredi.hu> <20070307082337.101759335@szeredi.hu> <20070307021518.3b1ff4a2.akpm@linux-foundation.org> Message-Id: From: Miklos Szeredi Date: Wed, 07 Mar 2007 11:31:59 +0100 Sender: owner-linux-mm@kvack.org Return-Path: To: akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, nickpiggin@yahoo.com.au, torvalds@linux-foundation.org List-ID: > (cc's reinstated) > > On Wed, 07 Mar 2007 09:09:50 +0100 Miklos Szeredi wrote: > > > There's a race in clear_page_dirty_for_io() that allows a page to have > > cleared PG_dirty, while being mapped read-write into the page table(s). > > I assume you refer to this: > > * FIXME! We still have a race here: if somebody > * adds the page back to the page tables in > * between the "page_mkclean()" and the "TestClearPageDirty()", > * we might have it mapped without the dirty bit set. > */ > if (page_mkclean(page)) > set_page_dirty(page); > if (TestClearPageDirty(page)) { > dec_zone_page_state(page, NR_FILE_DIRTY); > return 1; > } > Yes. > I guess the comment actually refers to a writefault after the > set_page_dirty() and before the TestClearPageDirty(). The fault handler > will run set_page_dirty() and will return to userspace to rerun the write. > The page then gets set pte-dirty but this thread of control will now make > the page !PageDirty() and will write it out. Yes. > With Nick's proposed lock-the-page-in-pagefaults patches, we have > lock_page() synchronisation between pagefaults and > clear_page_dirty_for_io() which I think will fix this. After a quick look, I don't think it does. It locks the page in do_no_page(), but not for the whole fault. In particular do_wp_page() is not affected. But I haven't yet looked closely at that patch, so I could be wrong. Miklos -- 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