From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751452AbdFHKd4 (ORCPT ); Thu, 8 Jun 2017 06:33:56 -0400 Received: from foss.arm.com ([217.140.101.70]:45658 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750752AbdFHKdz (ORCPT ); Thu, 8 Jun 2017 06:33:55 -0400 Date: Thu, 8 Jun 2017 11:34:02 +0100 From: Will Deacon To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, mark.rutland@arm.com, akpm@linux-foundation.org, kirill.shutemov@linux.intel.com, Punit.Agrawal@arm.com, mgorman@suse.de, steve.capper@arm.com Subject: Re: [PATCH 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered against prior accesses Message-ID: <20170608103402.GF6071@arm.com> References: <1496771916-28203-1-git-send-email-will.deacon@arm.com> <1496771916-28203-3-git-send-email-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 08, 2017 at 11:38:21AM +0200, Vlastimil Babka wrote: > On 06/06/2017 07:58 PM, Will Deacon wrote: > > page_ref_freeze and page_ref_unfreeze are designed to be used as a pair, > > wrapping a critical section where struct pages can be modified without > > having to worry about consistency for a concurrent fast-GUP. > > > > Whilst page_ref_freeze has full barrier semantics due to its use of > > atomic_cmpxchg, page_ref_unfreeze is implemented using atomic_set, which > > doesn't provide any barrier semantics and allows the operation to be > > reordered with respect to page modifications in the critical section. > > > > This patch ensures that page_ref_unfreeze is ordered after any critical > > section updates, by invoking smp_mb__before_atomic() prior to the > > atomic_set. > > > > Cc: "Kirill A. Shutemov" > > Acked-by: Steve Capper > > Signed-off-by: Will Deacon > > Undecided if it's really needed. This is IMHO not the classical case > from Documentation/core-api/atomic_ops.rst where we have to make > modifications visible before we let others see them? Here the one who is > freezing is doing it so others can't get their page pin and interfere > with the freezer's work. But maybe there are some (documented or not) > consistency guarantees to expect once you obtain the pin, that can be > violated, or they might be added later, so it would be safer to add the > barrier? The problem comes if the unfreeze is reordered so that it happens before the freezer has performed its work. For example, in migrate_huge_page_move_mapping: if (!page_ref_freeze(page, expected_count)) { spin_unlock_irq(&mapping->tree_lock); return -EAGAIN; } newpage->index = page->index; newpage->mapping = page->mapping; get_page(newpage); radix_tree_replace_slot(&mapping->page_tree, pslot, newpage); page_ref_unfreeze(page, expected_count - 1); then there's nothing stopping the CPU (and potentially the compiler) from reordering the unfreeze call so that it effectively becomes: if (!page_ref_freeze(page, expected_count)) { spin_unlock_irq(&mapping->tree_lock); return -EAGAIN; } page_ref_unfreeze(page, expected_count - 1); newpage->index = page->index; newpage->mapping = page->mapping; get_page(newpage); radix_tree_replace_slot(&mapping->page_tree, pslot, newpage); which then means that the freezer's work is carried out without the page being frozen. Will