From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f181.google.com (mail-ig0-f181.google.com [209.85.213.181]) by kanga.kvack.org (Postfix) with ESMTP id 8DB076B0036 for ; Tue, 16 Sep 2014 12:55:34 -0400 (EDT) Received: by mail-ig0-f181.google.com with SMTP id h3so421573igd.14 for ; Tue, 16 Sep 2014 09:55:34 -0700 (PDT) Received: from mail-ig0-x233.google.com (mail-ig0-x233.google.com [2607:f8b0:4001:c05::233]) by mx.google.com with ESMTPS id qr10si2258773igb.42.2014.09.16.09.55.33 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 16 Sep 2014 09:55:33 -0700 (PDT) Received: by mail-ig0-f179.google.com with SMTP id r10so429033igi.0 for ; Tue, 16 Sep 2014 09:55:33 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <1410811885-17267-1-git-send-email-andreslc@google.com> <54184078.4070505@redhat.com> Date: Tue, 16 Sep 2014 09:55:32 -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 9:52 AM, Andres Lagar-Cavilla wrote: Apologies to all. Resend as lists rejected my gmail-formatted version. Now on plain text. Won't happen again. > On Tue, Sep 16, 2014 at 6:51 AM, Paolo Bonzini wrote: >> >> Il 15/09/2014 22:11, Andres Lagar-Cavilla ha scritto: >> > + if (!locked) { >> > + BUG_ON(npages != -EBUSY); >> >> VM_BUG_ON perhaps? > > Sure. > >> >> > @@ -1177,9 +1210,15 @@ static int hva_to_pfn_slow(unsigned long addr, >> > bool *async, bool write_fault, >> > npages = get_user_page_nowait(current, current->mm, >> > addr, write_fault, page); >> > up_read(¤t->mm->mmap_sem); >> > - } else >> > - npages = get_user_pages_fast(addr, 1, write_fault, >> > - page); >> > + } else { >> > + /* >> > + * By now we have tried gup_fast, and possible async_pf, >> > and we >> > + * are certainly not atomic. Time to retry the gup, >> > allowing >> > + * mmap semaphore to be relinquished in the case of IO. >> > + */ >> > + npages = kvm_get_user_page_retry(current, current->mm, >> > addr, >> > + write_fault, page); >> >> This is a separate logical change. 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? > > > Nope. The intention was to pass FAULT_FLAG_RETRY to the vma fault handler > (without _NOWAIT). And once you do that, if you come back without holding > the mmap sem, you need to call yet again. > > 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). > >> >> 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. > > Have a look at how FAULT_FLAG_TRIED is used in e.g. arch/x86/mm/fault.c. > >> >> >> Apart from this, the patch looks good. The mm/ parts are minimal, so I >> think it's best to merge it through the KVM tree with someone's Acked-by. > > > Thanks! > Andres > >> >> >> Paolo > > > > > -- > Andres Lagar-Cavilla | Google Cloud Platform | andreslc@google.com | > 647-778-4380 -- Andres Lagar-Cavilla | Google Cloud Platform | andreslc@google.com | 647-778-4380 -- 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