From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756797AbaFIJKj (ORCPT ); Mon, 9 Jun 2014 05:10:39 -0400 Received: from mail-ig0-f171.google.com ([209.85.213.171]:58129 "EHLO mail-ig0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754900AbaFIJJO (ORCPT ); Mon, 9 Jun 2014 05:09:14 -0400 Date: Mon, 9 Jun 2014 02:09:10 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Vlastimil Babka cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Greg Thelen , Minchan Kim , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel Subject: Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner In-Reply-To: <53916BB0.3070001@suse.cz> Message-ID: References: <1401898310-14525-1-git-send-email-vbabka@suse.cz> <1401898310-14525-4-git-send-email-vbabka@suse.cz> <5390374E.5080708@suse.cz> <53916BB0.3070001@suse.cz> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 6 Jun 2014, Vlastimil Babka wrote: > >> > > diff --git a/mm/internal.h b/mm/internal.h > >> > > index 1a8a0d4..6aa1f74 100644 > >> > > --- a/mm/internal.h > >> > > +++ b/mm/internal.h > >> > > @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct > >> > > compact_control *cc, > >> > > * general, page_zone(page)->lock must be held by the caller to prevent > >> > > the > >> > > * page from being allocated in parallel and returning garbage as the > >> > > order. > >> > > * If a caller does not hold page_zone(page)->lock, it must guarantee > >> > > that the > >> > > - * page cannot be allocated or merged in parallel. > >> > > + * page cannot be allocated or merged in parallel. Alternatively, it must > >> > > + * handle invalid values gracefully, and use page_order_unsafe() below. > >> > > */ > >> > > static inline unsigned long page_order(struct page *page) > >> > > { > >> > > @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page > >> > > *page) > >> > > return page_private(page); > >> > > } > >> > > > >> > > +/* > >> > > + * Like page_order(), but for callers who cannot afford to hold the zone > >> > > lock, > >> > > + * and handle invalid values gracefully. ACCESS_ONCE is used so that if > >> > > the > >> > > + * caller assigns the result into a local variable and e.g. tests it for > >> > > valid > >> > > + * range before using, the compiler cannot decide to remove the variable > >> > > and > >> > > + * inline the function multiple times, potentially observing different > >> > > values > >> > > + * in the tests and the actual use of the result. > >> > > + */ > >> > > +static inline unsigned long page_order_unsafe(struct page *page) > >> > > +{ > >> > > + /* > >> > > + * PageBuddy() should be checked by the caller to minimize race > >> > > window, > >> > > + * and invalid values must be handled gracefully. > >> > > + */ > >> > > + return ACCESS_ONCE(page_private(page)); > >> > > +} > >> > > + > >> > > /* mm/util.c */ > >> > > void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma, > >> > > struct vm_area_struct *prev, struct rb_node *rb_parent); > >> > > >> > I don't like this change at all, I don't think we should have header > >> > functions that imply the context in which the function will be called. I > >> > think it would make much more sense to just do > >> > ACCESS_ONCE(page_order(page)) in the migration scanner with a comment. > >> > >> But that won't compile. It would have to be converted to a #define, unless > >> there's some trick I don't know. Sure I would hope this could be done cleaner > >> somehow. > >> > > > > Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner > > Hm but that's breaking the abstraction of page_order(). I don't know if it's > worse to create a new variant of page_order() or to do this. BTW, seems like > next_active_pageblock() in memory-hotplug.c should use this variant too. > The compiler seems free to disregard the access of a volatile object above because the return value of the inline function is unsigned long. What's the difference between unsigned long order = page_order_unsafe(page) and unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and the compiler being able to reaccess page_private() because the result is no longer volatile qualified? From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ie0-f179.google.com (mail-ie0-f179.google.com [209.85.223.179]) by kanga.kvack.org (Postfix) with ESMTP id 2CA4B6B0031 for ; Mon, 9 Jun 2014 05:09:15 -0400 (EDT) Received: by mail-ie0-f179.google.com with SMTP id rd18so5550622iec.10 for ; Mon, 09 Jun 2014 02:09:15 -0700 (PDT) Received: from mail-ie0-x22b.google.com (mail-ie0-x22b.google.com [2607:f8b0:4001:c03::22b]) by mx.google.com with ESMTPS id mk5si15799291igb.35.2014.06.09.02.09.14 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 09 Jun 2014 02:09:14 -0700 (PDT) Received: by mail-ie0-f171.google.com with SMTP id x19so275420ier.16 for ; Mon, 09 Jun 2014 02:09:14 -0700 (PDT) Date: Mon, 9 Jun 2014 02:09:10 -0700 (PDT) From: David Rientjes Subject: Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner In-Reply-To: <53916BB0.3070001@suse.cz> Message-ID: References: <1401898310-14525-1-git-send-email-vbabka@suse.cz> <1401898310-14525-4-git-send-email-vbabka@suse.cz> <5390374E.5080708@suse.cz> <53916BB0.3070001@suse.cz> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Greg Thelen , Minchan Kim , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel On Fri, 6 Jun 2014, Vlastimil Babka wrote: > >> > > diff --git a/mm/internal.h b/mm/internal.h > >> > > index 1a8a0d4..6aa1f74 100644 > >> > > --- a/mm/internal.h > >> > > +++ b/mm/internal.h > >> > > @@ -164,7 +164,8 @@ isolate_migratepages_range(struct zone *zone, struct > >> > > compact_control *cc, > >> > > * general, page_zone(page)->lock must be held by the caller to prevent > >> > > the > >> > > * page from being allocated in parallel and returning garbage as the > >> > > order. > >> > > * If a caller does not hold page_zone(page)->lock, it must guarantee > >> > > that the > >> > > - * page cannot be allocated or merged in parallel. > >> > > + * page cannot be allocated or merged in parallel. Alternatively, it must > >> > > + * handle invalid values gracefully, and use page_order_unsafe() below. > >> > > */ > >> > > static inline unsigned long page_order(struct page *page) > >> > > { > >> > > @@ -172,6 +173,23 @@ static inline unsigned long page_order(struct page > >> > > *page) > >> > > return page_private(page); > >> > > } > >> > > > >> > > +/* > >> > > + * Like page_order(), but for callers who cannot afford to hold the zone > >> > > lock, > >> > > + * and handle invalid values gracefully. ACCESS_ONCE is used so that if > >> > > the > >> > > + * caller assigns the result into a local variable and e.g. tests it for > >> > > valid > >> > > + * range before using, the compiler cannot decide to remove the variable > >> > > and > >> > > + * inline the function multiple times, potentially observing different > >> > > values > >> > > + * in the tests and the actual use of the result. > >> > > + */ > >> > > +static inline unsigned long page_order_unsafe(struct page *page) > >> > > +{ > >> > > + /* > >> > > + * PageBuddy() should be checked by the caller to minimize race > >> > > window, > >> > > + * and invalid values must be handled gracefully. > >> > > + */ > >> > > + return ACCESS_ONCE(page_private(page)); > >> > > +} > >> > > + > >> > > /* mm/util.c */ > >> > > void __vma_link_list(struct mm_struct *mm, struct vm_area_struct *vma, > >> > > struct vm_area_struct *prev, struct rb_node *rb_parent); > >> > > >> > I don't like this change at all, I don't think we should have header > >> > functions that imply the context in which the function will be called. I > >> > think it would make much more sense to just do > >> > ACCESS_ONCE(page_order(page)) in the migration scanner with a comment. > >> > >> But that won't compile. It would have to be converted to a #define, unless > >> there's some trick I don't know. Sure I would hope this could be done cleaner > >> somehow. > >> > > > > Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner > > Hm but that's breaking the abstraction of page_order(). I don't know if it's > worse to create a new variant of page_order() or to do this. BTW, seems like > next_active_pageblock() in memory-hotplug.c should use this variant too. > The compiler seems free to disregard the access of a volatile object above because the return value of the inline function is unsigned long. What's the difference between unsigned long order = page_order_unsafe(page) and unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and the compiler being able to reaccess page_private() because the result is no longer volatile qualified? -- 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