linux-kernel - Re: [PATCH 0/3] Guarantee faults for processes that call mmap(MAP

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Date:	Thu, 8 May 2008 11:48:22 +1000
From:	David Gibson <dwg@....ibm.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, dean@...tic.org, apw@...dowen.org,
	linux-kernel@...r.kernel.org, wli@...omorphy.com,
	andi@...stfloor.org, kenchen@...gle.com, agl@...ibm.com
Subject: Re: [PATCH 0/3] Guarantee faults for processes that call
	mmap(MAP_PRIVATE) on hugetlbfs v2

On Wed, May 07, 2008 at 08:38:26PM +0100, Mel Gorman wrote:
> MAP_SHARED mappings on hugetlbfs reserve huge pages at mmap() time.
> This guarantees all future faults against the mapping will succeed.
> This allows local allocations at first use improving NUMA locality whilst
> retaining reliability.
> 
> MAP_PRIVATE mappings do not reserve pages. This can result in an application
> being SIGKILLed later if a huge page is not available at fault time. This
> makes huge pages usage very ill-advised in some cases as the unexpected
> application failure cannot be detected and handled as it is immediately fatal.
> Although an application may force instantiation of the pages using mlock(),
> this may lead to poor memory placement and the process may still be killed
> when performing COW.
> 
> This patchset introduces a reliability guarantee for the process which creates
> a private mapping, i.e. the process that calls mmap() on a hugetlbfs file
> successfully.  The first patch of the set is purely mechanical code move to
> make later diffs easier to read. The second patch will guarantee faults up
> until the process calls fork(). After patch two, as long as the child keeps
> the mappings, the parent is no longer guaranteed to be reliable. Patch
> 3 guarantees that the parent will always successfully COW by unmapping
> the pages from the child in the event there are insufficient pages in the
> hugepage pool in allocate a new page, be it via a static or dynamic pool.

I don't think patch 3 is a good idea.  It's a fair bit of code to
implement a pretty bizarre semantic that I really don't think is all
that useful.  Patches 1-2 are already sufficient to cover the
fork()/exec() case and a fair proportion of fork()/minor
frobbing/exit() cases.  If the child also needs to write the hugepage
area, chances are it's doing real work and we care about its
reliability too.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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