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Date: Tue, 26 Jun 2018 09:43:44 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for
 large mapping
Message-ID: <20180626074344.GZ2458@hirez.programming.kicks-ass.net>
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To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Michal Hocko <mhocko@kernel.org>, Nadav Amit <nadav.amit@gmail.com>, Matthew Wilcox <willy@infradead.org>, ldufour@linux.vnet.ibm.com, Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@redhat.com>, acme@kernel.org, alexander.shishkin@linux.intel.com, jolsa@redhat.com, namhyung@kernel.org, "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>, linux-kernel@vger.kernel.org

On Mon, Jun 25, 2018 at 05:06:23PM -0700, Yang Shi wrote:
> By looking this deeper, we may not be able to cover all the unmapping range
> for VM_DEAD, for example, if the start addr is in the middle of a vma. We
> can't set VM_DEAD to that vma since that would trigger SIGSEGV for still
> mapped area.
> 
> splitting can't be done with read mmap_sem held, so maybe just set VM_DEAD
> to non-overlapped vmas. Access to overlapped vmas (first and last) will
> still have undefined behavior.

Acquire mmap_sem for writing, split, mark VM_DEAD, drop mmap_sem. Acquire
mmap_sem for reading, madv_free drop mmap_sem. Acquire mmap_sem for
writing, free everything left, drop mmap_sem.

?

Sure, you acquire the lock 3 times, but both write instances should be
'short', and I suppose you can do a demote between 1 and 2 if you care.