From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 18 Jun 2008 15:04:36 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [PATCH] migration_entry_wait fix. Message-Id: <20080618150436.dca5eb75.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <200806181535.58036.nickpiggin@yahoo.com.au> References: <20080611225945.4da7bb7f.akpm@linux-foundation.org> <20080618101349.db4d5205.kamezawa.hiroyu@jp.fujitsu.com> <20080618105435.de10d6bc.kamezawa.hiroyu@jp.fujitsu.com> <200806181535.58036.nickpiggin@yahoo.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Nick Piggin Cc: Daisuke Nishimura , Andrew Morton , Rik van Riel , Lee Schermerhorn , Kosaki Motohiro , Nick Piggin , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-testers@vger.kernel.org, "hugh@veritas.com" List-ID: On Wed, 18 Jun 2008 15:35:57 +1000 Nick Piggin wrote: > On Wednesday 18 June 2008 11:54, KAMEZAWA Hiroyuki wrote: > > On Wed, 18 Jun 2008 10:13:49 +0900 > > > > KAMEZAWA Hiroyuki wrote: > > > + if (!page_cache_get_speculative()) > > > + goto out; > > > > This is obviously buggy....sorry..quilt refresh miss.. > > > > == > > In speculative page cache lookup protocol, page_count(page) is set to 0 > > while radix-tree modification is going on, truncation, migration, etc... > > These tend to all happen while the page is locked, and in particular > while the page does not have any references other than the current > code path and the pagecache. So no page tables should point to it. > > So migration_entry_wait should not find pages with a refcount of zero. > > > > While page migration, a page fault to page under migration should wait > > unlock_page() and migration_entry_wait() waits for the page from its > > pte entry. It does get_page() -> wait_on_page_locked() -> put_page() now. > > > > In page migration, page_freeze_refs() -> page_unfreeze_refs() is called. > > > > Here, page_unfreeze_refs() expects page_count(page) == 0 and panics > > if page_count(page) != 0. To avoid this, we shouldn't touch page_count() > > if it is zero. This patch uses page_cache_get_speculative() to avoid > > the panic. > > At any rate, page_cache_get_speculative() should not be used for this > purpose, but for when we _really_ don't have any references to a page. > Then, I got NAK. what should I do ? (This fix is not related to lock_page() problem.) If I read your advice correctly, we shouldn't use lock_page() here. Before speculative page cache, page_table_entry of a page under migration has a pte entry which encodes pfn as special pte entry. and wait for the end of page migration by lock_page(). Maybe we just go back to user-land and makes it to do page-fault again is better ? Thanks, -Kame -- 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