From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qc0-f176.google.com (mail-qc0-f176.google.com [209.85.216.176]) by kanga.kvack.org (Postfix) with ESMTP id C39836B0035 for ; Tue, 16 Sep 2014 16:51:50 -0400 (EDT) Received: by mail-qc0-f176.google.com with SMTP id x13so707935qcv.35 for ; Tue, 16 Sep 2014 13:51:50 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id x10si20495267qci.7.2014.09.16.13.51.48 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 16 Sep 2014 13:51:49 -0700 (PDT) Date: Tue, 16 Sep 2014 22:51:10 +0200 From: Radim =?utf-8?B?S3LEjW3DocWZ?= Subject: Re: [PATCH] kvm: Faults which trigger IO release the mmap_sem Message-ID: <20140916205110.GA1273@potion.brq.redhat.com> References: <1410811885-17267-1-git-send-email-andreslc@google.com> <54184078.4070505@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <54184078.4070505@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Paolo Bonzini Cc: Andres Lagar-Cavilla , 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 2014-09-15 13:11-0700, Andres Lagar-Cavilla: > +int kvm_get_user_page_retry(struct task_struct *tsk, struct mm_struct *mm, The suffix '_retry' is not best suited for this. On first reading, I imagined we will be retrying something from before, possibly calling it in a loop, but we are actually doing the first and last try in one call. Hard to find something that conveys our lock-dropping mechanic, '_polite' is my best candidate at the moment. > + int flags = FOLL_TOUCH | FOLL_HWPOISON | (FOLL_HWPOISON wasn't used before, but it's harmless.) 2014-09-16 15:51+0200, Paolo Bonzini: > Il 15/09/2014 22:11, Andres Lagar-Cavilla ha scritto: > > @@ -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 ^ (If we really tried get_user_pages_fast, we wouldn't be here, so I'd prepend two underscores here as well.) > > + * 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? I believe so as well. (Looking at get_user_pages_fast and __get_user_pages_fast made my abstraction detector very sad.) > 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"). Not sure if that would help to understand the goal ... > 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. I would prefer to have the last hunk in a separate patch, but still, Acked-by: Radim KrA?mA!A? -- 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