From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id 58F0E6B002D for ; Thu, 22 Mar 2018 11:55:01 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id o23so4555716wrc.9 for ; Thu, 22 Mar 2018 08:55:01 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id 33si855021ede.244.2018.03.22.08.54.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Mar 2018 08:54:59 -0700 (PDT) Received: from pps.filterd (m0098416.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w2MFs9mg141154 for ; Thu, 22 Mar 2018 11:54:58 -0400 Received: from e06smtp14.uk.ibm.com (e06smtp14.uk.ibm.com [195.75.94.110]) by mx0b-001b2d01.pphosted.com with ESMTP id 2gvcfpswn3-1 (version=TLSv1.2 cipher=AES256-SHA256 bits=256 verify=NOT) for ; Thu, 22 Mar 2018 11:54:58 -0400 Received: from localhost by e06smtp14.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 22 Mar 2018 15:54:56 -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> <18a727fd-f006-9fae-d9ca-74b9004f0a8b@linux.vnet.ibm.com> <20180322154055.GB28468@bombadil.infradead.org> From: Laurent Dufour Date: Thu, 22 Mar 2018 16:54:52 +0100 MIME-Version: 1.0 In-Reply-To: <20180322154055.GB28468@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <0442fb0e-3da3-3f23-ce4d-0f6cbc3eac9a@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Wilcox Cc: Yang Shi , Michal Hocko , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 22/03/2018 16:40, Matthew Wilcox wrote: > On Thu, Mar 22, 2018 at 04:32:00PM +0100, Laurent Dufour wrote: >> 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. > > The page fault handler will walk the VMA tree to find the correct > VMA and then find that the VMA is marked as deleted. If it assumes > that the VMA has been deleted because of munmap(), then it can raise > SIGSEGV immediately. But if the VMA is marked as deleted because of > mmap(MAP_FIXED), it must wait until the new VMA is in place. I'm wondering if such a complexity is required. If the user space process try to access the page being overwritten through mmap(MAP_FIXED) by another thread, there is no guarantee that it will manipulate the *old* page or *new* one. I'd think this is up to the user process to handle that concurrency. What needs to be guaranteed is that once mmap(MAP_FIXED) returns the old page are no more there, which is done through the mmap_sem and PTE locking. > I think I was wrong to describe VMAs as being *deleted*. I think we > instead need the concept of a *locked* VMA that page faults will block on. > Conceptually, it's a per-VMA rwsem, but I'd use a completion instead of > an rwsem since the only reason to write-lock the VMA is because it is > being deleted. Such a lock would only makes sense in the case of mmap(MAP_FIXED) since when the VMA is removed there is no need to wait. Isn't it ?