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

[Davidlohr Bueso]

On Wed, 2013-08-07 at 11:03 +1000, David Gibson wrote:
> 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.
> 

Exactly, which is why I suggest minimizing the lock contention until we
do have such a proposal.

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


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