On Mon, Aug 11, 2014 at 04:35:00PM +0200, Jan Kara wrote: > On Mon 11-08-14 10:13:08, Matthew Wilcox wrote: > > On Mon, Aug 11, 2014 at 10:51:47AM +0200, Jan Kara wrote: > > > So I'm afraid we'll have to find some other way to synchronize > > > page faults and truncate / punch hole in DAX. > > > > What if we don't? If we hit the race (which is vanishingly unlikely with > > real applications), the consequence is simply that after a truncate, a > > file may be left with one or two blocks allocated somewhere after i_size. > > As I understand it, that's not a real problem; they're temporarily > > unavailable for allocation but will be freed on file removal or the next > > truncation of that file. > You mean if you won't have any locking between page fault and truncate? > You can have: > a) extending truncate making forgotten blocks with non-zeros visible > b) filesystem corruption due to doubly used blocks (block will be freed > from the truncated file and thus can be reallocated but it will still be > accessible via mmap from the truncated file). > > So not a good idea. Not *no* locking ... just no locking around get_block, like in v7. So check i_size, call get_block, lock i_mmap_mutex, re-check i_size, insert mapping if i_size is OK, drop i_mmap_mutex. As long as get_block() has enough locking of its own against set_size and concurrent calls to get_block(), I don't think we can get visible non-zeroes or double allocation. > > I'm also still considering the possibility of having truncate-down block > > until all mmaps that extend after the new i_size have been removed ... > Hum, I'm not sure how you would do that with current locking scheme and > wait for all page faults on that range to finish but maybe you have some > good idea :) While it can be blocked with i_dio_count currently, this would be a more complicated thing to do ... -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
