From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by kanga.kvack.org (Postfix) with ESMTP id BB4AF6B0003 for ; Tue, 3 Jul 2018 07:34:58 -0400 (EDT) Received: by mail-ed1-f70.google.com with SMTP id v19-v6so836505eds.3 for ; Tue, 03 Jul 2018 04:34:58 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id o34-v6si939011edb.336.2018.07.03.04.34.56 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 03 Jul 2018 04:34:56 -0700 (PDT) Date: Tue, 3 Jul 2018 13:34:53 +0200 From: Michal Hocko Subject: Re: [RFC v3 PATCH 4/5] mm: mmap: zap pages with read mmap_sem for large mapping Message-ID: <20180703113453.GJ16767@dhcp22.suse.cz> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180703091911.hhxhnqpeqb2kn42x@kshutemo-mobl1> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" 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 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. -- Michal Hocko SUSE Labs