On Fri 04-10-13 16:42:19, KOSAKI Motohiro wrote: > (10/4/13 4:31 PM), KOSAKI Motohiro wrote: > >(10/2/13 4:29 PM), Jan Kara wrote: > >>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). > > > >If target page point to anon or private mapping pages, get_user_pages_fast() > >is fork unsafe. O_DIRECT man pages describe a bit about this. > > > > > >see http://man7.org/linux/man-pages/man2/open.2.html > > > >> O_DIRECT I/Os should never be run concurrently with the fork(2) > >> system call, if the memory buffer is a private mapping (i.e., any > >> mapping created with the mmap(2) MAP_PRIVATE flag; this includes > >> memory allocated on the heap and statically allocated buffers). Any > >> such I/Os, whether submitted via an asynchronous I/O interface or > >> from another thread in the process, should be completed before > >> fork(2) is called. Failure to do so can result in data corruption > >> and undefined behavior in parent and child processes. This > >> restriction does not apply when the memory buffer for the O_DIRECT > >> I/Os was created using shmat(2) or mmap(2) with the MAP_SHARED flag. > >> Nor does this restriction apply when the memory buffer has been > >> advised as MADV_DONTFORK with madvise(2), ensuring that it will not > >> be available to the child after fork(2). > > IMHO, get_user_pages_fast() should be renamed to get_user_pages_quirk(). Its > semantics is not equal to get_user_pages(). When someone simply substitute > get_user_pages() to get_user_pages_fast(), they might see huge trouble. I forgot about this speciality (and actually comments didn't remind me :(). But thinking about this some more get_user_pages_fast() seems as save as get_user_pages() in presence of threads sharing mm, doesn't it? Because while get_user_pages() are working, other thread can happilly trigger COW on some of the pages and thus get_user_pages() can return pages some of which are invisible in our mm by the time get_user_pages() returns. So although in practice I agree problems of get_user_pages_fast() with fork(2) are more visible, in essence they are still present with clone(2) and get_user_pages(). 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>