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@xxxxxxxxxxxxxxxxxxxxx> > > > > > > > > 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>