Re: [PATCH] hugepage: allow parallelization of the hugepage fault path

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Jul 19, 2013 at 02:24:15PM -0700, Davidlohr Bueso wrote:
> On Fri, 2013-07-19 at 17:14 +1000, David Gibson wrote:
> > On Thu, Jul 18, 2013 at 05:42:35PM +0900, Joonsoo Kim wrote:
> > > On Wed, Jul 17, 2013 at 12:50:25PM -0700, Davidlohr Bueso wrote:
> > > > From: David Gibson <david@gibson.dropbear.id.au>
> > > > 
> > > > At present, the page fault path for hugepages is serialized by a
> > > > single mutex. This is used to avoid spurious out-of-memory conditions
> > > > when the hugepage pool is fully utilized (two processes or threads can
> > > > race to instantiate the same mapping with the last hugepage from the
> > > > pool, the race loser returning VM_FAULT_OOM).  This problem is
> > > > specific to hugepages, because it is normal to want to use every
> > > > single hugepage in the system - with normal pages we simply assume
> > > > there will always be a few spare pages which can be used temporarily
> > > > until the race is resolved.
> > > > 
> > > > Unfortunately this serialization also means that clearing of hugepages
> > > > cannot be parallelized across multiple CPUs, which can lead to very
> > > > long process startup times when using large numbers of hugepages.
> > > > 
> > > > This patch improves the situation by replacing the single mutex with a
> > > > table of mutexes, selected based on a hash, which allows us to know
> > > > which page in the file we're instantiating. For shared mappings, the
> > > > hash key is selected based on the address space and file offset being faulted.
> > > > Similarly, for private mappings, the mm and virtual address are used.
> > > > 
> > > 
> > > Hello.
> > > 
> > > With this table mutex, we cannot protect region tracking structure.
> > > See below comment.
> > > 
> > > /*
> > >  * Region tracking -- allows tracking of reservations and instantiated pages
> > >  *                    across the pages in a mapping.
> > >  *
> > >  * The region data structures are protected by a combination of the mmap_sem
> > >  * and the hugetlb_instantion_mutex.  To access or modify a region the caller
> > >  * must either hold the mmap_sem for write, or the mmap_sem for read and
> > >  * the hugetlb_instantiation mutex:
> > >  *
> > >  *      down_write(&mm->mmap_sem);
> > >  * or
> > >  *      down_read(&mm->mmap_sem);
> > >  *      mutex_lock(&hugetlb_instantiation_mutex);
> > >  */
> > 
> > Ugh.  Who the hell added that.  I guess you'll need to split of
> > another mutex for that purpose, afaict there should be no interaction
> > with the actual, intended purpose of the instantiation mutex.
> 
> This was added in commit 84afd99b. One way to go would be to add a
> spinlock to protect changes to the regions - however reading the
> changelog, and based on David's previous explanation for the
> instantiation mutex, I don't see why it was added. In fact several
> places modify regions without holding the instantiation mutex, ie:
> hugetlb_reserve_pages()
> 
> Am I missing something here?

hugetlb_reserve_pages() is called with down_write(mmap_sem),
so fault flow which require down_read(mmap_sem) cannot interfere
to change the region.

Thanks.

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