From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f170.google.com (mail-ig0-f170.google.com [209.85.213.170]) by kanga.kvack.org (Postfix) with ESMTP id B3D4F6B0036 for ; Tue, 16 Sep 2014 14:42:19 -0400 (EDT) Received: by mail-ig0-f170.google.com with SMTP id l13so4059252iga.3 for ; Tue, 16 Sep 2014 11:42:19 -0700 (PDT) Received: from mail-ig0-x22d.google.com (mail-ig0-x22d.google.com [2607:f8b0:4001:c05::22d]) by mx.google.com with ESMTPS id qo6si2547495igb.23.2014.09.16.11.42.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 16 Sep 2014 11:42:18 -0700 (PDT) Received: by mail-ig0-f173.google.com with SMTP id l13so6118260iga.6 for ; Tue, 16 Sep 2014 11:42:18 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <54188179.7010705@redhat.com> References: <1410811885-17267-1-git-send-email-andreslc@google.com> <54184078.4070505@redhat.com> <54188179.7010705@redhat.com> Date: Tue, 16 Sep 2014 11:42:17 -0700 Message-ID: Subject: Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem From: Andres Lagar-Cavilla Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: To: Paolo Bonzini 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 On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini wrote: > Il 16/09/2014 18:52, Andres Lagar-Cavilla ha scritto: >> Was this: >> >> down_read(&mm->mmap_sem); >> npages = get_user_pages(NULL, mm, addr, 1, 1, 0, NULL, NULL); >> up_read(&mm->mmap_sem); >> >> the intention rather than get_user_pages_fast? > > I meant the intention of the original author, not yours. Yes, in all likelihood. I hope! > >> By that point in the call chain I felt comfortable dropping the _fast. >> All paths that get there have already tried _fast (and some have tried >> _NOWAIT). > > Yes, understood. > >> 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). Thanks Andres > > Paolo -- 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