Re: [120/136] mm: munlock use follow_page

[Hugh Dickins]

On Thu, 1 Oct 2009, Greg KH wrote:
> 2.6.31-stable review patch.  If anyone has any objections, please let us know.
> 
> ------------------
> From: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
> 
> commit 408e82b78bcc9f1b47c76e833c3df97f675947de upstream.
> 
> Hiroaki Wakabayashi points out that when mlock() has been interrupted
> by SIGKILL, the subsequent munlock() takes unnecessarily long because
> its use of __get_user_pages() insists on faulting in all the pages
> which mlock() never reached.
> 
> It's worse than slowness if mlock() is terminated by Out Of Memory kill:
> the munlock_vma_pages_all() in exit_mmap() insists on faulting in all the
> pages which mlock() could not find memory for; so innocent bystanders are
> killed too, and perhaps the system hangs.
> 
> __get_user_pages() does a lot that's silly for munlock(): so remove the
> munlock option from __mlock_vma_pages_range(), and use a simple loop of
> follow_page()s in munlock_vma_pages_range() instead; ignoring absent
> pages, and not marking present pages as accessed or dirty.
> 
> (Change munlock() to only go so far as mlock() reached?  That does not
> work out, given the convention that mlock() claims complete success even
> when it has to give up early - in part so that an underlying file can be
> extended later, and those pages locked which earlier would give SIGBUS.)

Thanks a lot for including this fix in 2.6.31.2.

If I'd done my homework, I'd be asking you to include the equivalent
in 2.6.30.9 (it's irrelevant to 2.6.27.36); but because of intervening
changes, although it would appear to apply there, it would be wrong.

And it's more than a one-liner, so I'd need to retest properly.
If there's ever a call for a 2.6.30.10, I ought to supply you with
the proper patch; but for now, unless others disagree, let's not
rush it for 2.6.30.9.

Hugh
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