Re: [PATCH 17/18] mm, hugetlb: retry if we fail to allocate ahugepage with use_reserve

[David Gibson]

On Tue, Aug 06, 2013 at 05:18:44PM -0700, Davidlohr Bueso wrote:
> On Mon, 2013-08-05 at 16:36 +0900, Joonsoo Kim wrote:
> > > Any mapping that doesn't use the reserved pool, not just
> > > MAP_NORESERVE.  For example, if a process makes a MAP_PRIVATE mapping,
> > > then fork()s then the mapping is instantiated in the child, that will
> > > not draw from the reserved pool.
> > > 
> > > > Should we ensure them to allocate the last hugepage?
> > > > They map a region with MAP_NORESERVE, so don't assume that their requests
> > > > always succeed.
> > > 
> > > If the pages are available, people get cranky if it fails for no
> > > apparent reason, MAP_NORESERVE or not.  They get especially cranky if
> > > it sometimes fails and sometimes doesn't due to a race condition.
> > 
> > Hello,
> > 
> > Hmm... Okay. I will try to implement another way to protect race condition.
> > Maybe it is the best to use a table mutex :)
> > Anyway, please give me a time, guys.
> 
> So another option is to take the mutex table patchset for now as it
> *does* improve things a great deal, then, when ready, get rid of the
> instantiation lock all together.

We still don't have a solid proposal for doing that. Joonsoo Kim's
patchset misses cases (non reserved mappings).  I'm also not certain
there aren't a few edge cases which can lead to even reserved mappings
failing, and if that happens the patches will lead to livelock.

Getting rid of the instantiation mutex is a lot harder than it appears.

-- 
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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