linux-kernel - Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap

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Date:   Mon, 25 Jun 2018 11:14:55 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Nadav Amit <nadav.amit@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        ldufour@...ux.vnet.ibm.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping

On Fri 22-06-18 18:01:08, Yang Shi wrote:
> Yes, this is true but I guess what Yang Shi meant was that an userspace
> > > access racing with munmap is not well defined. You never know whether
> > > you get your data, #PTF or SEGV because it depends on timing. The user
> > > visible change might be that you lose content and get zero page instead
> > > if you hit the race window while we are unmapping which was not possible
> > > before. But whouldn't such an access pattern be buggy anyway? You need
> > > some form of external synchronization AFAICS.
> > > 
> > > But maybe some userspace depends on "getting right data or get SEGV"
> > > semantic. If we have to preserve that then we can come up with a VM_DEAD
> > > flag set before we tear it down and force the SEGV on the #PF path.
> > > Something similar we already do for MMF_UNSTABLE.
> > 
> > Set VM_DEAD with read mmap_sem held? It should be ok since this is the
> > only place to set this flag for this unique special case.
> 
> BTW, it looks the vm flags have used up in 32 bit. If we really need
> VM_DEAD, it should be for both 32-bit and 64-bit.

Do we really need any special handling for 32b? Who is going to create
GB mappings for all this to be worth doing?

-- 
Michal Hocko
SUSE Labs