From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f200.google.com (mail-qt0-f200.google.com [209.85.216.200]) by kanga.kvack.org (Postfix) with ESMTP id F2BF16B0012 for ; Thu, 22 Mar 2018 11:32:26 -0400 (EDT) Received: by mail-qt0-f200.google.com with SMTP id n51so5817837qta.9 for ; Thu, 22 Mar 2018 08:32:26 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id b49si5491028qte.189.2018.03.22.08.32.25 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Mar 2018 08:32:26 -0700 (PDT) Received: from pps.filterd (m0098396.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w2MFW2ob020696 for ; Thu, 22 Mar 2018 11:32:25 -0400 Received: from e06smtp11.uk.ibm.com (e06smtp11.uk.ibm.com [195.75.94.107]) by mx0a-001b2d01.pphosted.com with ESMTP id 2gvda06ym4-1 (version=TLSv1.2 cipher=AES256-SHA256 bits=256 verify=NOT) for ; Thu, 22 Mar 2018 11:32:22 -0400 Received: from localhost by e06smtp11.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 22 Mar 2018 15:32:03 -0000 Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section References: <1521581486-99134-1-git-send-email-yang.shi@linux.alibaba.com> <1521581486-99134-2-git-send-email-yang.shi@linux.alibaba.com> <20180321130833.GM23100@dhcp22.suse.cz> <20180321172932.GE4780@bombadil.infradead.org> <20180321224631.GB3969@bombadil.infradead.org> From: Laurent Dufour Date: Thu, 22 Mar 2018 16:32:00 +0100 MIME-Version: 1.0 In-Reply-To: <20180321224631.GB3969@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <18a727fd-f006-9fae-d9ca-74b9004f0a8b@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox , Yang Shi Cc: Michal Hocko , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 21/03/2018 23:46, Matthew Wilcox wrote: > On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote: >> Marking vma as deleted sounds good. The problem for my current approach is >> the concurrent page fault may succeed if it access the not yet unmapped >> section. Marking deleted vma could tell page fault the vma is not valid >> anymore, then return SIGSEGV. >> >>> does not care; munmap will need to wait for the existing munmap operation >> >> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right? > > The other thing about MAP_FIXED that we'll need to handle is unmapping > conflicts atomically. Say a program has a 200GB mapping and then > mmap(MAP_FIXED) another 200GB region on top of it. So I think page faults > are also going to have to wait for deleted vmas (then retry the fault) > rather than immediately raising SIGSEGV. Regarding the page fault, why not relying on the PTE locking ? When munmap() will unset the PTE it will have to held the PTE lock, so this will serialize the access. If the page fault occurs before the mmap(MAP_FIXED), the page mapped will be removed when mmap(MAP_FIXED) would do the cleanup. Fair enough.