From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 427266B0003 for ; Thu, 22 Mar 2018 12:46:59 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id 2so4901108pft.4 for ; Thu, 22 Mar 2018 09:46:59 -0700 (PDT) Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com. [115.124.30.130]) by mx.google.com with ESMTPS id b9si5241705pff.169.2018.03.22.09.46.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 22 Mar 2018 09:46:57 -0700 (PDT) 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> <0442fb0e-3da3-3f23-ce4d-0f6cbc3eac9a@linux.vnet.ibm.com> <20180322160547.GC28468@bombadil.infradead.org> <55ac947f-fd77-3754-ebfe-30d458c54403@linux.vnet.ibm.com> From: Yang Shi Message-ID: Date: Thu, 22 Mar 2018 09:46:38 -0700 MIME-Version: 1.0 In-Reply-To: <55ac947f-fd77-3754-ebfe-30d458c54403@linux.vnet.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Laurent Dufour , Matthew Wilcox Cc: Michal Hocko , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 3/22/18 9:18 AM, Laurent Dufour wrote: > > On 22/03/2018 17:05, Matthew Wilcox wrote: >> On Thu, Mar 22, 2018 at 04:54:52PM +0100, Laurent Dufour wrote: >>> On 22/03/2018 16:40, Matthew Wilcox wrote: >>>> On Thu, Mar 22, 2018 at 04:32:00PM +0100, Laurent Dufour wrote: >>>>> 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. >> Right; but it must return one or the other, it can't segfault. > Good point, I missed that... > >>> 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. >> Yes, and allowing the fault handler to return the *old* page risks the >> old page being reinserted into the page tables after the unmapping task >> has done its work. > The PTE locking should prevent that. > >> It's *really* rare to page-fault on a VMA which is in the middle of >> being replaced. Why are you trying to optimise it? > I was not trying to optimize it, but to not wait in the page fault handler. > This could become tricky in the case the VMA is removed once mmap(MAP_FIXED) is > done and before the waiting page fault got woken up. This means that the > removed VMA structure will have to remain until all the waiters are woken up > which implies ref_count or similar. We may not need ref_count. After removing "locked-for-deletion" vmas when mmap(MAP_FIXED) is done, just wake up page fault to re-lookup vma, then it will find the new vma installed by mmap(MAP_FIXED), right? I'm not sure if completion can do this or not since I'm not quite familiar with it :-( Yang > >>>> 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 ? >> I can't think of another reason. I suppose we could mark the VMA as >> locked-for-deletion or locked-for-replacement and have the SIGSEGV happen >> early. But I'm not sure that optimising for SIGSEGVs is a worthwhile >> use of our time. Just always have the pagefault sleep for a deleted VMA.