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Subject: Re: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap_sem in
 munmap
References: <1534358990-85530-1-git-send-email-yang.shi@linux.alibaba.com>
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From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <45a5ff36-d53d-9ec3-f869-1b1b7a6de5cb@intel.com>
Date: Wed, 22 Aug 2018 14:42:27 -0700
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To: "Kirill A. Shutemov" <kirill@shutemov.name>, Yang Shi <yang.shi@linux.alibaba.com>
Cc: Vlastimil Babka <vbabka@suse.cz>, mhocko@kernel.org, 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, linux-mm@kvack.org, linux-kernel@vger.kernel.org

On 08/22/2018 02:10 PM, Kirill A. Shutemov wrote:
>> For x86, mpx_notify_unmap() looks finally zap the VM_MPX vmas in bound table
>> range with zap_page_range() and doesn't update vm flags, so it sounds ok to
>> me since vmas have been detached, nobody can find those vmas. But, I'm not
>> familiar with the details of mpx, maybe Kirill could help to confirm this?
> I don't see anything obviously dependent on down_write() in
> mpx_notify_unmap(), but Dave should know better.

We need mmap_sem for write in mpx_notify_unmap().

Its job is to clean up bounds tables, but bounds tables are dynamically
allocated and destroyed by the kernel.  When we destroy a table, we also
destroy the VMA for the bounds table *itself*, separate from the VMA
being unmapped.

But, this code is very likely to go away soon.  If it's causing a
problem for you, let me know and I'll see if I can get to removing it
faster.