On Sun, Oct 16, 2022 at 05:06:56PM +0200, Fabio M. De Francesco wrote: > The use of kmap() and kmap_atomic() are being deprecated in favor of > kmap_local_page(). > > There are two main problems with kmap(): (1) It comes with an overhead as > the mapping space is restricted and protected by a global lock for > synchronization and (2) it also requires global TLB invalidation when the > kmap’s pool wraps and it might block when the mapping space is fully > utilized until a slot becomes available. > > With kmap_local_page() the mappings are per thread, CPU local, can take > page faults, and can be called from any context (including interrupts). > It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore, > the tasks can be preempted and, when they are scheduled to run again, the > kernel virtual addresses are restored and still valid. > > Since its use in fs/aio.c is safe everywhere, it should be preferred. > > Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in > fs/aio.c. > > Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel > with HIGHMEM64GB enabled. I was just looking over this code and made the same kmap -> kmap_local change, but you've done a more complete version - nice. For context, I was the one who added the kmap() call, because copy_to_user() can sleep - anything else at the time would've been more awkward, and highmem machines were already on the way out. But kmap_local is perfect here :) Reviewed-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
