On Tue, Dec 13, 2022 at 05:13:10PM +0100, Ard Biesheuvel wrote: > kmap_atomic() is used to create short-lived mappings of pages that may > not be accessible via the kernel direct map. This is only needed on > 32-bit architectures that implement CONFIG_HIGHMEM, but it can be used > on 64-bit other architectures too, where the returned mapping is simply > the kernel direct address of the page. > > However, kmap_atomic() does not support migration on CONFIG_HIGHMEM > configurations, due to the use of per-CPU kmap slots, and so it disables > preemption on all architectures, not just the 32-bit ones. This implies > that all scatterwalk based crypto routines essentially execute with > preemption disabled all the time, which is less than ideal. > > So let's switch scatterwalk_map/_unmap and the shash/ahash routines to > kmap_local() instead, which serves a similar purpose, but without the > resulting impact on preemption on architectures that have no need for > CONFIG_HIGHMEM. > > Cc: Eric Biggers <ebiggers@xxxxxxxxxx> > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> > Cc: "Elliott, Robert (Servers)" <elliott@xxxxxxx> > Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > --- > crypto/ahash.c | 4 ++-- > crypto/shash.c | 4 ++-- > include/crypto/scatterwalk.h | 4 ++-- > 3 files changed, 6 insertions(+), 6 deletions(-) > Thanks Ard, this looks good to me, especially given the broader effort to replace kmap_atomic() with kmap_local_page() in the kernel. One question: should the kmap_atomic() in crypto/skcipher.c be replaced as well? - Eric
