From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751937AbaIQHny (ORCPT ); Wed, 17 Sep 2014 03:43:54 -0400 Received: from mail-wg0-f48.google.com ([74.125.82.48]:34489 "EHLO mail-wg0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751203AbaIQHnw (ORCPT ); Wed, 17 Sep 2014 03:43:52 -0400 Message-ID: <54193BB2.8010500@redhat.com> Date: Wed, 17 Sep 2014 09:43:46 +0200 From: Paolo Bonzini User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.0 MIME-Version: 1.0 To: Andres Lagar-Cavilla CC: Gleb Natapov , Rik van Riel , Peter Zijlstra , Mel Gorman , Andy Lutomirski , Andrew Morton , Andrea Arcangeli , Sasha Levin , Jianyu Zhan , Paul Cassella , Hugh Dickins , Peter Feiner , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem References: <1410811885-17267-1-git-send-email-andreslc@google.com> <54184078.4070505@redhat.com> <54188179.7010705@redhat.com> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Il 16/09/2014 20:42, Andres Lagar-Cavilla ha scritto: > On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini wrote: >>> I think a first patch should introduce kvm_get_user_page_retry ("Retry a >>> fault after a gup with FOLL_NOWAIT.") and the second would add >>> FOLL_TRIED ("This properly relinquishes mmap semaphore if the >>> filemap/swap has to wait on page lock (and retries the gup to completion >>> after that"). >>> >>> That's not what FOLL_TRIED does. The relinquishing of mmap semaphore is >>> done by this patch minus the FOLL_TRIED bits. FOLL_TRIED will let the >>> fault handler (e.g. filemap) know that we've been there and waited on >>> the IO already, so in the common case we won't need to redo the IO. >> >> Yes, that's not what FOLL_TRIED does. But it's the difference between >> get_user_pages and kvm_get_user_page_retry, right? > > Unfortunately get_user_pages does not expose the param (int > *nonblocking) that __gup will use to set FAULT_FLAG_ALLOW_RETRY. So > that's one difference. The second difference is that kvm_gup_retry > will call two times if necessary (the second without _RETRY but with > _TRIED). Yeah, that's how it is in your patch. I can see that. What I'm saying is that your patch is two changes in one: 1) do not use gup_fast in hva_to_pfn_slow, instead use gup as in async_pf_execute. This change can already introduce a function called kvm_get_user_page_retry, and can already use it in async_pf_execute and hva_to_pfn_slow 2) introduce the two-phase RETRY + TRIED mechanism in kvm_get_user_page_retry, so that the mmap semaphore is relinquished properly if the filemap or swap has to wait on the page lock. I would prefer to split it in two patches. Is it clearer now? Paolo