On Wed 02-10-13 09:20:09, Christoph Hellwig wrote: > On Wed, Oct 02, 2013 at 04:27:41PM +0200, Jan Kara wrote: > > Hello, > > > > In my quest for changing locking around page faults to make things easier for > > filesystems I found out get_user_pages() users could use a cleanup. The > > knowledge about necessary locking for get_user_pages() is in tons of places in > > drivers and quite a few of them actually get it wrong (don't have mmap_sem when > > calling get_user_pages() or hold mmap_sem when calling copy_from_user() in the > > surrounding code). Rather often this actually doesn't seem necessary. This > > patch series converts lots of places to use either get_user_pages_fast() > > or a new simple wrapper get_user_pages_unlocked() to remove the knowledge > > of mmap_sem from the drivers. I'm still looking into converting a few remaining > > drivers (most notably v4l2) which are more complex. > > Even looking over the kerneldoc comment next to it I still fail to > understand when you'd want to use get_user_pages_fast and when not. AFAIU get_user_pages_fast() should be used 1) if you don't need any special get_user_pages() arguments (like calling it for mm of a different process, forcing COW, or similar). 2) you don't expect pages to be unmapped (then get_user_pages_fast() is actually somewhat slower because it walks page tables twice). Honza -- Jan Kara <jack@xxxxxxx> 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>