On Thu, Feb 14, 2019 at 01:55:19PM -0800, 'Todd Kjos' via kernel-team wrote: > On Thu, Feb 14, 2019 at 1:25 PM Joel Fernandes <joelaf@xxxxxxxxxx> wrote: > > > > On Thu, Feb 14, 2019 at 03:53:54PM -0500, Joel Fernandes wrote: > > > On Thu, Feb 14, 2019 at 3:42 PM Todd Kjos <tkjos@xxxxxxxxxx> wrote: > > > > > > > > On Thu, Feb 14, 2019 at 11:45 AM Joel Fernandes <joelaf@xxxxxxxxxx> wrote: > > > [snip] > > > > > > + * check_buffer() - verify that buffer/offset is safe to access > > > > > > + * @alloc: binder_alloc for this proc > > > > > > + * @buffer: binder buffer to be accessed > > > > > > + * @offset: offset into @buffer data > > > > > > + * @bytes: bytes to access from offset > > > > > > + * > > > > > > + * Check that the @offset/@bytes are within the size of the given > > > > > > + * @buffer and that the buffer is currently active and not freeable. > > > > > > + * Offsets must also be multiples of sizeof(u32). The kernel is > > > > > > > > > > In all callers of binder_alloc_copy_user_to_buffer, the alignment of offsets > > > > > is set to sizeof(void *). Then shouldn't this function check for sizeof(void *) > > > > > alignment instead of u32? > > > > > > > > But there are other callers of check_buffer() later in the series that > > > > don't require pointer-size alignment. u32 alignment is consistent with > > > > the alignment requirements of the binder driver before this change. > > > > The copy functions don't actually need to insist on alignment, but > > > > these binder buffer objects have always used u32 alignment which has > > > > been checked in the driver. If user code misaligned it, then errors > > > > are returned. The alignment checks are really to be consistent with > > > > previous binder driver behavior. > > > > > > Got it, thanks. > > > > One more thing I wanted to ask is, kmap() will now cause global lock > > contention because of using spin_lock due to kmap_high(). > > > > Previously the binder driver was made to not use global lock (as you had > > done). Now these paths will start global locking on 32-bit architectures. > > Would that degrade performance? > > There was a lot of concern about 32-bit performance both for > lock-contention and the cost of map/unmap operations. Of course, > 32-bit systems are also where the primary win is -- namely avoiding > vmalloc space depletion. So there was a several months-long evaluation > period on 32-bit devices by a silicon vendor who did a lot of testing > across a broad set of benchmarks / workloads to verify the performance > costs are acceptable. We also ran tests to try to exhaust the kmap > space with multiple large buffers. > > The testing did find that there is some performance degradation for > large buffer transfers, but there are no cases where this > significantly impacted a meaningful user workload. > > > > > Are we not using kmap_atomic() in this patch because of any concern that the > > kmap fixmap space is limited and may run out? > > We're not using the atomic version here since we can't guarantee that > the loop will be atomic since we are calling copy_from_user(). Later > in the series, other cases do use kmap_atomic(). Got it, thanks for all the clarifications, - Joel _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
