From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.5 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C2A74C433B4 for ; Mon, 3 May 2021 16:02:35 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C226B61185 for ; Mon, 3 May 2021 16:02:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C226B61185 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 31A556B0070; Mon, 3 May 2021 12:02:32 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 2A25A6B0071; Mon, 3 May 2021 12:02:32 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 12EC36B0072; Mon, 3 May 2021 12:02:32 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0223.hostedemail.com [216.40.44.223]) by kanga.kvack.org (Postfix) with ESMTP id E60D16B0070 for ; Mon, 3 May 2021 12:02:31 -0400 (EDT) Received: from smtpin29.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 99D3E6D7B for ; Mon, 3 May 2021 16:02:31 +0000 (UTC) X-FDA: 78100387302.29.00DEABD Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf09.hostedemail.com (Postfix) with ESMTP id 5E18360006D6 for ; Mon, 3 May 2021 16:02:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=JfhcAPftKZ8AsOlG154NgUsbwjQGFIEm1AMmoheMFkA=; b=OwP0/5CV1pUfYfQ4i6uG8wSwaV tST4mGakUlvxX86IJ4ouiltvqG3RiOnvY4JtD8UKaM0e+UOasps7MuU2lc9ATfTWJBXaogQgwgFwQ s/8woBTekFdK6LRupbN65TkOODriNgX6aA+S0IrYQKLHk0RH7HKU/KC/QBhkiv5DmXez33Uq2t1CR +/whsp9RlNLLptL29Ud3tZogzarCbl6Jk+9fKYBV4Xs/gm2IDNShIaz0PCq/YT3cFscGA86UrL+r0 jAaTrsPBxsJjALIwRIQUohNh8DPkvf5F0gUIQaR3t18ZhVNnbhhAT0Tpd5+rKSaV2Riz/odS2Vrdy UKPCZMWQ==; Received: from willy by casper.infradead.org with local (Exim 4.94 #2 (Red Hat Linux)) id 1ldazc-00FHow-8Z; Mon, 03 May 2021 16:00:39 +0000 Date: Mon, 3 May 2021 17:00:20 +0100 From: Matthew Wilcox To: Liam Howlett Cc: Michel Lespinasse , "maple-tree@lists.infradead.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Andrew Morton , Song Liu , Davidlohr Bueso , "Paul E . McKenney" , Laurent Dufour , David Rientjes , Axel Rasmussen , Suren Baghdasaryan , Vlastimil Babka , Rik van Riel , Peter Zijlstra , Michel Lespinasse Subject: Re: [PATCH 38/94] mm/gup: Add mm_populate_vma() for use when the vma is known Message-ID: <20210503160020.GC1847222@casper.infradead.org> References: <20210428153542.2814175-1-Liam.Howlett@Oracle.com> <20210428153542.2814175-39-Liam.Howlett@Oracle.com> <20210501051330.GC5188@lespinasse.org> <20210503155352.74xqhqgizaagdro5@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210503155352.74xqhqgizaagdro5@revolver> Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b="OwP0/5CV"; spf=none (imf09.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org; dmarc=none X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 5E18360006D6 X-Stat-Signature: 8aipnukzw5jcybg15om1e3zd41fuq4mx Received-SPF: none (infradead.org>: No applicable sender policy available) receiver=imf09; identity=mailfrom; envelope-from=""; helo=casper.infradead.org; client-ip=90.155.50.34 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1620057742-686459 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, May 03, 2021 at 03:53:58PM +0000, Liam Howlett wrote: > * Michel Lespinasse [210501 01:13]: > > On Wed, Apr 28, 2021 at 03:36:08PM +0000, Liam Howlett wrote: > > > When a vma is known, avoid calling mm_populate to search for the vma to > > > populate. > > > > > > Signed-off-by: Liam R. Howlett > > > --- > > > mm/gup.c | 20 ++++++++++++++++++++ > > > mm/internal.h | 4 ++++ > > > 2 files changed, 24 insertions(+) > > > > > > diff --git a/mm/gup.c b/mm/gup.c > > > index c3a17b189064..48fe98ab0729 100644 > > > --- a/mm/gup.c > > > +++ b/mm/gup.c > > > @@ -1468,6 +1468,26 @@ long populate_vma_page_range(struct vm_area_struct *vma, > > > NULL, NULL, locked); > > > } > > > > > > +/* > > > + * mm_populate_vma() - Populate a single range in a single vma. > > > + * @vma: The vma to populate. > > > + * @start: The start address to populate > > > + * @end: The end address to stop populating > > > + * > > > + * Note: Ignores errors. > > > + */ > > > +void mm_populate_vma(struct vm_area_struct *vma, unsigned long start, > > > + unsigned long end) > > > +{ > > > + struct mm_struct *mm = current->mm; > > > + int locked = 1; > > > + > > > + mmap_read_lock(mm); > > > + populate_vma_page_range(vma, start, end, &locked); > > > + if (locked) > > > + mmap_read_unlock(mm); > > > +} > > > + > > > > This seems like a nonsensical API at first glance - VMAs that are found > > in the vma tree might be modified, merged, split, or freed at any time > > if the mmap lock is not held, so the API can not be safely used. I think > > this applies to maple tree vmas just as much as it did for rbtree vmas ? > > This is correct - it cannot be used without having the mmap_sem lock. > This is a new internal mm code API and is used to avoid callers that use > mm_populate() on a range that is known to be in a single VMA and already > have that VMA. So instead of re-walking the tree to re-find the VMAs, > this function can be used with the known VMA and range. > > It is used as described in patch 39 and 40 of this series. In patch 39, what you do is: 1 Take the mmap_sem for write 2 do stuff 3 Drop the mmap_sem 4 Call mm_populate_vm() with the vma, which takes the mmap_sem for read The problem is that between 3 & 4, a racing thread might cause us to free the vma and so we've now passed a bogus pointer into mm_populate_vm(). What we need instead is to downgrade the mmap_sem from write to read at step 3, so the vma is guaranteed to still be good.