On Tue, Jul 16, 2013 at 08:01:46PM +1000, David Gibson wrote: > On Tue, Jul 16, 2013 at 02:34:24PM +0900, Joonsoo Kim wrote: > > On Mon, Jul 15, 2013 at 09:51:21PM -0400, Rik van Riel wrote: > > > On 07/15/2013 03:24 AM, David Gibson wrote: > > > >On Sun, Jul 14, 2013 at 08:16:44PM -0700, Davidlohr Bueso wrote: > > > > > > >>>Reading the existing comment, this change looks very suspicious to me. > > > >>>A per-vma mutex is just not going to provide the necessary exclusion, is > > > >>>it? (But I recall next to nothing about these regions and > > > >>>reservations.) > > > > > > > >A per-VMA lock is definitely wrong. I think it handles one form of > > > >the race, between threads sharing a VM on a MAP_PRIVATE mapping. > > > >However another form of the race can and does occur between different > > > >MAP_SHARED VMAs in the same or different processes. I think there may > > > >be edge cases involving mremap() and MAP_PRIVATE that will also be > > > >missed by a per-VMA lock. > > > > > > > >Note that the libhugetlbfs testsuite contains tests for both PRIVATE > > > >and SHARED variants of the race. > > > > > > Can we get away with simply using a mutex in the file? > > > Say vma->vm_file->mapping->i_mmap_mutex? > > > > I totally agree with this approach :) > > > > > > > > That might help with multiple processes initializing > > > multiple shared memory segments at the same time, and > > > should not hurt the case of a process mapping its own > > > hugetlbfs area. > > > > > > It might have the potential to hurt when getting private > > > copies on a MAP_PRIVATE area, though. I have no idea > > > how common it is for multiple processes to MAP_PRIVATE > > > the same hugetlbfs file, though... > > > > Currently, getting private copies on a MAP_PRIVATE area is also > > serialized by hugetlb_instantiation_mutex. > > How do we get worse with your approach? > > > > BTW, we have one race problem related to hugetlb_instantiation_mutex. > > It is not right protection for region structure handling. We map the > > area without holding a hugetlb_instantiation_mutex, so there is > > race condition between mapping a new area and faulting the other area. > > Am I missing? > > The hugetlb_instantiation_mutex has nothing to do with protecting > region structures. It exists only to address one very specific and > frequently misunderstood race. Yes, it was introduced for that purpose, but, currently, it is also used for protecting region structure. You can see below comment in mm/hugetlb.c * 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. > > -- > 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 -- 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>