Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754736Ab1FBNah (ORCPT ); Thu, 2 Jun 2011 09:30:37 -0400 Received: from mx1.redhat.com ([209.132.183.28]:1032 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753396Ab1FBNaf (ORCPT ); Thu, 2 Jun 2011 09:30:35 -0400 Date: Thu, 2 Jun 2011 15:29:54 +0200 From: Andrea Arcangeli To: Mel Gorman Cc: Minchan Kim , akpm@linux-foundation.org, Ury Stankevich , KOSAKI Motohiro , linux-kernel@vger.kernel.org, linux-mm@kvack.org, stable@kernel.org Subject: Re: [PATCH] mm: compaction: Abort compaction if too many pages are isolated and caller is asynchronous Message-ID: <20110602132954.GC19505@random.random> References: <20110531121620.GA3490@barrios-laptop> <20110531122437.GJ19505@random.random> <20110531133340.GB3490@barrios-laptop> <20110531141402.GK19505@random.random> <20110601005747.GC7019@csn.ul.ie> <20110601175809.GB7306@suse.de> <20110601191529.GY19505@random.random> <20110601214018.GC7306@suse.de> <20110601233036.GZ19505@random.random> <20110602010352.GD7306@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110602010352.GD7306@suse.de> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2055 Lines: 54 On Thu, Jun 02, 2011 at 02:03:52AM +0100, Mel Gorman wrote: > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 2d29c9a..65fa251 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -631,12 +631,14 @@ static int __do_huge_pmd_anonymous_page(struct mm_struct *mm, > entry = mk_pmd(page, vma->vm_page_prot); > entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma); > entry = pmd_mkhuge(entry); > + > /* > - * The spinlocking to take the lru_lock inside > - * page_add_new_anon_rmap() acts as a full memory > - * barrier to be sure clear_huge_page writes become > - * visible after the set_pmd_at() write. > + * Need a write barrier to ensure the writes from > + * clear_huge_page become visible before the > + * set_pmd_at > */ > + smp_wmb(); > + On x86 at least this is noop because of the spin_lock(&page_table_lock) after clear_huge_page. But I'm not against adding this in case other archs supports THP later. But smp_wmb() is optimized away at build time by cpp so this can't possibly help if you're reproducing !SMP. > page_add_new_anon_rmap(page, vma, haddr); > set_pmd_at(mm, haddr, pmd, entry); > prepare_pmd_huge_pte(pgtable, mm); > @@ -753,6 +755,13 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm, > > pmdp_set_wrprotect(src_mm, addr, src_pmd); > pmd = pmd_mkold(pmd_wrprotect(pmd)); > + > + /* > + * Write barrier to make sure the setup for the PMD is fully visible > + * before the set_pmd_at > + */ > + smp_wmb(); > + > set_pmd_at(dst_mm, addr, dst_pmd, pmd); > prepare_pmd_huge_pte(pgtable, dst_mm); This part seems superfluous to me, it's also noop for !SMP. Only wmb() would stay. the pmd is perfectly fine to stay in a register, not even a compiler barrier is needed, even less a smp serialization. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/