From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f72.google.com (mail-pl0-f72.google.com [209.85.160.72]) by kanga.kvack.org (Postfix) with ESMTP id 548E16B0003 for ; Tue, 3 Jul 2018 08:14:58 -0400 (EDT) Received: by mail-pl0-f72.google.com with SMTP id 39-v6so1148815ple.6 for ; Tue, 03 Jul 2018 05:14:58 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id a137-v6sor270840pfa.112.2018.07.03.05.14.56 for (Google Transport Security); Tue, 03 Jul 2018 05:14:56 -0700 (PDT) Date: Tue, 3 Jul 2018 15:14:50 +0300 From: "Kirill A. Shutemov" Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping Message-ID: <20180703121450.6aytgmssmf26bgos@kshutemo-mobl1> References: <1530311985-31251-1-git-send-email-yang.shi@linux.alibaba.com> <1530311985-31251-5-git-send-email-yang.shi@linux.alibaba.com> <20180702123350.dktmzlmztulmtrae@kshutemo-mobl1> <20180702124928.GQ19043@dhcp22.suse.cz> <20180703081205.3ue5722pb3ko4g2w@kshutemo-mobl1> <20180703082718.GF16767@dhcp22.suse.cz> <20180703091911.hhxhnqpeqb2kn42x@kshutemo-mobl1> <20180703113453.GJ16767@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180703113453.GJ16767@dhcp22.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Yang Shi , willy@infradead.org, ldufour@linux.vnet.ibm.com, akpm@linux-foundation.org, peterz@infradead.org, mingo@redhat.com, acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, tglx@linutronix.de, hpa@zytor.com, linux-mm@kvack.org, x86@kernel.org On Tue, Jul 03, 2018 at 01:34:53PM +0200, Michal Hocko wrote: > On Tue 03-07-18 12:19:11, Kirill A. Shutemov wrote: > > On Tue, Jul 03, 2018 at 10:27:18AM +0200, Michal Hocko wrote: > > > On Tue 03-07-18 11:12:05, Kirill A. Shutemov wrote: > > > > On Mon, Jul 02, 2018 at 02:49:28PM +0200, Michal Hocko wrote: > > > > > On Mon 02-07-18 15:33:50, Kirill A. Shutemov wrote: > > > > > [...] > > > > > > I probably miss the explanation somewhere, but what's wrong with allowing > > > > > > other thread to re-populate the VMA? > > > > > > > > > > We have discussed that earlier and it boils down to how is racy access > > > > > to munmap supposed to behave. Right now we have either the original > > > > > content or SEGV. If we allow to simply madvise_dontneed before real > > > > > unmap we could get a new page as well. There might be (quite broken I > > > > > would say) user space code that would simply corrupt data silently that > > > > > way. > > > > > > > > Okay, so we add a lot of complexity to accommodate broken userspace that > > > > may or may not exist. Is it right? :) > > > > > > I would really love to do the most simple and obious thing > > > > > > diff --git a/mm/mmap.c b/mm/mmap.c > > > index 336bee8c4e25..86ffb179c3b5 100644 > > > --- a/mm/mmap.c > > > +++ b/mm/mmap.c > > > @@ -2811,6 +2811,8 @@ EXPORT_SYMBOL(vm_munmap); > > > SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len) > > > { > > > profile_munmap(addr); > > > + if (len > LARGE_NUMBER) > > > + do_madvise(addr, len, MADV_DONTNEED); > > > return vm_munmap(addr, len); > > > } > > > > > > but the argument that current semantic of good data or SEGV on > > > racing threads is no longer preserved sounds valid to me. Remember > > > optimizations shouldn't eat your data. How do we ensure that we won't > > > corrupt data silently? > > > > +linux-api > > > > Frankly, I don't see change in semantics here. > > > > Code that has race between munmap() and page fault would get intermittent > > SIGSEGV before and after the approach with simple MADV_DONTNEED. > > prior to this patch you would either get an expected content (if you > win the race) or SEGV otherwise. With the above change you would get a > third state - a fresh new page (zero page) if you lost the race half > way. That sounds like a change of a long term semantic. > > How much that matters is of course a question. Userspace is known to do > the most unexpected things you never even dreamed of. I bet nobody would notice the difference. Let's go the simple way. The price to protect against *theoretical* broken userspace is too high. -- Kirill A. Shutemov