From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail172.messagelabs.com (mail172.messagelabs.com [216.82.254.3]) by kanga.kvack.org (Postfix) with SMTP id 8B5535F0001 for ; Wed, 15 Apr 2009 04:22:21 -0400 (EDT) From: Nick Piggin Subject: Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page Date: Wed, 15 Apr 2009 18:22:32 +1000 References: <20090414143252.GE28265@random.random> <200904150042.15653.nickpiggin@yahoo.com.au> <20090415165431.AC4C.A69D9226@jp.fujitsu.com> In-Reply-To: <20090415165431.AC4C.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200904151822.33478.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org To: KOSAKI Motohiro Cc: Andrea Arcangeli , LKML , Linus Torvalds , Andrew Morton , Jeff Moyer , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, Hugh Dickins List-ID: On Wednesday 15 April 2009 18:05:54 KOSAKI Motohiro wrote: > Hi > > > On Wednesday 15 April 2009 00:32:52 Andrea Arcangeli wrote: > > > On Wed, Apr 15, 2009 at 12:26:34AM +1000, Nick Piggin wrote: > > > > Andrea: I didn't veto that set_bit change of yours as such. I just > > > > > > I know you didn't ;) > > > > > > > noted there could be more atomic operations. Actually I would > > > > welcome more comparison between our two approaches, but they seem > > > > > > Agree about the welcome of comparison, it'd be nice to measure it the > > > enterprise workloads that showed the gup_fast gain in the first place. > > > > I think we should be able to ask IBM to run some tests, provided > > they still have machines available to do so. Although I don't want > > to waste their time so we need to have something that has got past > > initial code review and has a chance of being merged. > > > > If we get that far, then I can ask them to run tests definitely. > > Oh, it seem very charming idea. > Nick, I hope to help your patch's rollup. It makes good comparision, I think. > Is there my doable thing? Well, I guess review and testing. There are few possibilities for reducing the cases where we have to de-cow (or increasing the cases where we can WP-on-fork), which I'd like to experiment with, but I don't know how much it will help... > And, I changed my patch. > How about this? I added simple twice check. > > because, both do_wp_page and try_to_unmap_one grab ptl. then, > page-fault routine can't change pte while try_to_unmap nuke pte. Hmm, > @@ -790,7 +796,19 @@ static int try_to_unmap_one(struct page > > /* Nuke the page table entry. */ > flush_cache_page(vma, address, page_to_pfn(page)); > - pteval = ptep_clear_flush_notify(vma, address, pte); > + pteval = ptep_clear_flush(vma, address, pte); > + > + if (!migration) { > + /* re-check */ > + if (PageSwapCache(page) && > + page_count(page) != page_mapcount(page) + 2) { > + /* We lose race against get_user_pages_fast() */ > + set_pte_at(mm, address, pte, pteval); > + ret = SWAP_FAIL; > + goto out_unmap; > + } > + } > + mmu_notifier_invalidate_page(vma->vm_mm, address); Hmm, in the case of powerpc-style gup_fast where the arch does not send IPIs to flush TLBs, either the speculative reference there should find the pte cleared, or the page_count check here should find the speculative reference. In the case of CPUs that do send IPIs and have x86-style gup_fast, the TLB flush should ensure all gup_fast()s that could have seen the pte will complete before we check page_count. Yes I think it might work. -- 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