From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 11 Apr 2006 08:55:29 -0700 (PDT) From: Christoph Lameter Subject: Re: Page Migration: Make do_swap_page redo the fault In-Reply-To: <1144767501.5160.12.camel@localhost.localdomain> Message-ID: References: <1144767501.5160.12.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org Return-Path: To: Lee Schermerhorn Cc: Hugh Dickins , akpm@osdl.org, linux-mm@kvack.org List-ID: On Tue, 11 Apr 2006, Lee Schermerhorn wrote: > > Hmmm... The increased count is also an argument against having to check > > for the race in do_swap_page(). So maybe Lee's lazy migration patchset > > should also be fine without these checks and there is actually no need > > to rely on the ptes not being the same. > > May still be some work in do_swap_page(). The unmap has already > occurred. In the general case [support for migrating pages w/ > 1 pte > mapping], two or more tasks could race faulting the cache pte. IMO one > should perform the migration [replacing old page in cache with new > page], others should block and then use the new page to resolve their > own faults. I think this means a check and then at least another cache > lookup. Maybe redo the fault, as Christoph has said. > > Don't know about direct migration. In direct migration the reference counts avoid these problems. The worst case scenario that we have imagined so far assumed that migration occurs after do_swap_cache has done a lookup of the old page in the swap_cache. The check that we had in there was envisioned to protect against the case of migration happening after the lookup_swap_cache() occurred. As Hugh noted: lookup_swap_cache() is increasing the page count under tree_lock. direct migration also checks page count under the tree lock. So migration cannot occur after the lookup_swap_cache() has been done and returned the old page. If lookup_swap_cache returns the new page then we will block on lock_page() until migration has finished. -- 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