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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] proc: pagemap: Hold mmap_sem during page walk
Cc: kosaki.motohiro@jp.fujitsu.com, Matt Mackall <mpm@selenic.com>,
       San Mehat <san@google.com>, linux-kernel@vger.kernel.org,
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> So there is a very real reason for that pattern existing. It's just that 
> that reason has nothing to do with locking the thing into the page tables. 
> That is absolutely NOT guaranteed by the get_user_pages() physical page 
> pinning (munmap() is an extreme example of this, but I think swapout will 
> clear it too in try_to_unmap_one().

Right. To increment page->count only affect vmscan to prevent free page.
Not prevent pageout I/O nor page unmapping from a process.

Thanks.