On Wed, 14 Dec 2016, Michal Hocko wrote: > On Tue 13-12-16 08:33:34, Alan Stern wrote: > > On Tue, 13 Dec 2016, Michal Hocko wrote: > > > > > > > That being said, what ep_write_iter does sounds quite stupit. It just > > > > > allocates a large continuous buffer which seems to be under user > > > > > control... Aka no good! It should do that per pages or something like > > > > > that. Something worth fixing > > > > > > > > It's not important enough to make the driver do all this work. If > > > > users want to send large amounts of data, they can send it a page at a > > > > time (or something like that). > > > > > > Is it really necessary to allocate the full iov_iter_count? Why cannot > > > we process the from buffer one page at a time? > > > > We could (although one page is really too small -- USB 3.1 can transfer > > 800 KB per ms so we ought to handle at least 128 KB at a time). > > Is there any problem to submit larger transfers without having the > buffer physically contiguous? Async I/O would be rather awkward; it would have to use a work queue routine. But it could be done. And we would still end up allocating the same total space (more actually, because we would need to store the scatter-gather table too). It just wouldn't be contiguous. > > But > > turn the argument around: If the user wants to transfer that much data, > > why can't he _submit_ it one page at a time? > > Not sure I understand. > > > > > If you really want to prevent the driver from attempting to allocate a > > > > large buffer, all that's needed is an upper limit on the total size. > > > > For example, 64 KB. > > > > > > Well, my point was that it is not really hard to imagine to deplete > > > larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are > > > still causing the OOM killer and chances are that a controlled flood of > > > these requests could completely DoS the system. > > > > Putting a limit on the total size of a single transfer would prevent > > this. > > Dunno, putting a limit to the user visible interface sounds wrong to me. In practice, I think the data transfer sizes tend to be not very large. But I could be wrong about that. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
