linux-kernel - [PATCH 0/2] RFC: page munlock issues when breaking up COW

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Message-Id: <1291880979-16309-1-git-send-email-walken@google.com>
Date:	Wed,  8 Dec 2010 23:49:37 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Nick Piggin <npiggin@...nel.dk>, linux-mm@...ck.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH 0/2] RFC: page munlock issues when breaking up COW

I'm sending this up as RFC only as I've only done minimal testing so far -
I would actually be particularly interested in comments about any corner
cases I must make sure to test for...

It looks like there is a race in the do_wp_page() code that munlocks the
old page after breaking up COW. The pte still points to that old page,
so I don't see that we are protected against vmscan mlocking back the
page right away. This can be easily worked around by moving that code to
the end of do_wp_page(), after the pte has been pointed to the new page.

Also, the corresponding code in __do_fault() seems entirely unnecessary,
since there was never a pte pointing to the old page in our vma.

I found this by code inspection only, and while I believe I understand
this code well by now, there is always the possibility that I may have
missed something. I hope Nick can comment, since he wrote this part of
the code.

Michel Lespinasse (2):
  mlock: fix race when munlocking pages in do_wp_page()
  mlock: do not munlock pages in __do_fault()

 mm/memory.c |   32 ++++++++++++--------------------
 1 files changed, 12 insertions(+), 20 deletions(-)

-- 
1.7.3.1

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