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

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

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);
 */

Thanks.

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