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Date: Mon, 09 Sep 2002 12:14:42 -0700
From: Andrew Morton <akpm@digeo.com>
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To: Daniel Phillips <phillips@arcor.de>
CC: imran.badr@cavium.com, root@chaos.analogic.com,
       "'David S. Miller'" <davem@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: Calculating kernel logical address ..
References: <01a801c25831$913c5fd0$9e10a8c0@IMRANPC> <20020909190328Z16576-16323+113@humbolt.nl.linux.org>
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Daniel Phillips wrote:
> 
> ...
> > down(&current->mm->mmap_sem) would help.
> 
> Not for anon pages, and how do you know whether it's anon or not before
> looking at the page, which may be free by the time you look at it?
> In other words, mm->page_table_lock is the one, because it's required
> for unmapping a pte, and any mapped page will be forced to hold a count
> increment until it gets past that lock.  Without this lock, the results
> of pte_page are unstable.

The caller of get_user_pages() needs to hold mmap_sem for reading
to prevent the vmas from going away.  get_user_pages() does the
right thing wrt page_table_lock.  (As a quick peek at the code
would reveal...)
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