linux-kernel - Re: [PATCH v2 2/5] userfaultfd: non-cooperative: add event for memory unmaps

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Message-Id: <20170131163928.4e4e05fbd93358d9b937d79b@linux-foundation.org>
Date:   Tue, 31 Jan 2017 16:39:28 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] userfaultfd: non-cooperative: add event for
 memory unmaps

On Fri, 27 Jan 2017 20:44:30 +0200 Mike Rapoport <rppt@...ux.vnet.ibm.com> wrote:

> When a non-cooperative userfaultfd monitor copies pages in the background,
> it may encounter regions that were already unmapped. Addition of
> UFFD_EVENT_UNMAP allows the uffd monitor to track precisely changes in the
> virtual memory layout.
> 
> Since there might be different uffd contexts for the affected VMAs, we
> first should create a temporary representation for the unmap event for each
> uffd context and then notify them one by one to the appropriate userfault
> file descriptors.
> 
> The event notification occurs after the mmap_sem has been released.

I was going to bug you about not updating
Documentation/vm/userfaultfd.txt but the UFFD_FEATURE flags aren't
documented?