From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another From: Lee Schermerhorn In-Reply-To: References: <1226409701-14831-3-git-send-email-ieidus@redhat.com> <20081111114555.eb808843.akpm@linux-foundation.org> <20081111210655.GG10818@random.random> <20081111221753.GK10818@random.random> <20081111231722.GR10818@random.random> <20081112022701.GT10818@random.random> <20081112173258.GX10818@random.random> Content-Type: text/plain Date: Wed, 12 Nov 2008 17:09:03 -0500 Message-Id: <1226527744.7560.93.camel@lts-notebook> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Christoph Lameter Cc: Andrea Arcangeli , Andrew Morton , Izik Eidus , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kvm@vger.kernel.org, chrisw@redhat.com, avi@redhat.com, izike@qumranet.com List-ID: On Wed, 2008-11-12 at 14:27 -0600, Christoph Lameter wrote: > On Wed, 12 Nov 2008, Andrea Arcangeli wrote: > > > On Tue, Nov 11, 2008 at 09:10:45PM -0600, Christoph Lameter wrote: > > > get_user_pages() cannot get to it since the pagetables have already been > > > modified. If get_user_pages runs then the fault handling will occur > > > which will block the thread until migration is complete. > > > > migrate.c does nothing for ptes pointing to swap entries and > > do_swap_page won't wait for them either. Assume follow_page in > > If a anonymous page is a swap page then it has a mapping. > migrate_page_move_mapping() will lock the radix tree and ensure that no > additional reference (like done by do_swap_page) is established during > migration. So, it's Nick's reference freezing you asked about in response to my mail that prevents do_swap_page() from getting another reference on the page in the swap cache just after migrate_page_move_mapping() checks the ref count and replaces the slot with new swap pte. Radix tree lock just prevents other threads from modifying the slot, right? [Hmmm, looks like we need to update the reference to "write lock" in the comments on the 'deref_slot() and _replace_slot() definitions in radix-tree.h.] Therefore, do_swap_page() will either get the old page and raise the ref before migration check, or it will [possibly loop in find_get_page() and then] get the new page. Migration will bail out, for this pass anyway, in the former case. In the second case, do_swap_page() will wait on the new page lock until migration completes, deferring any direct IO. Or am I still missing something? > > > However it's not exactly the same bug as the one in fork, I was > > talking about before, it's also not o_direct specific. Still > > So far I have seen wild ideas not bugs. Maybe not so wild, given the complexity of these interactions... Later, Lee -- 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