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? Thanks. > > -- > All rights reversed > > -- > 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> -- 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>