On Wed 17-08-16 14:25:56, Ross Zwisler wrote: > On Tue, Aug 16, 2016 at 11:28:16AM +0200, Jan Kara wrote: > > On Mon 15-08-16 13:09:16, Ross Zwisler wrote: > > > DAX radix tree locking currently locks entries based on the unique > > > combination of the 'mapping' pointer and the pgoff_t 'index' for the entry. > > > This works for PTEs, but as we move to PMDs we will need to have all the > > > offsets within the range covered by the PMD to map to the same bit lock. > > > To accomplish this, lock based on the 'slot' pointer in the radix tree > > > instead of [mapping, index]. > > > > I'm not convinced this is safe. What makes the slot pointer still valid > > after you drop tree_lock? At least radix_tree_shrink() or > > radix_tree_expand() could move your slot without letting the waiter know > > and he would be never woken. > > > > Honza > > Yep, you're right, thanks for catching that. > > Given that we can't rely on 'slot' being stable, my next idea is to use a > combination of [mapping, index], but tweak 'index' so that it's always the > beginning of the entry. So for 4k entries we'd leave it alone, but for 2MiB > entries we'd mask it down to the appropriate 2MiB barrier. > > Let me hack on that for a bit, unless you've a better idea. No, that's what I'd do as well. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- 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>