mm: optimize copy_page_to/from_iter_iovec
copy_page_to_iter_iovec() and copy_page_from_iter_iovec() copy some data
to userspace or from userspace. These functions have a fast path where
they map a page using kmap_atomic and a slow path where they use kmap.
kmap is slower than kmap_atomic, so the fast path is preferred.
However, on kernels without highmem support, kmap just calls page_address,
so there is no need to avoid kmap. On kernels without highmem support,
the fast path just increases code size (and cache footprint) and it
doesn't improve copy performance in any way.
This patch enables the fast path only if CONFIG_HIGHMEM is defined.
Code size reduced by this patch:
x86 (without highmem) 928
x86-64 960
sparc64 848
alpha 1136
pa-risc 1200
Link: http://lkml.kernel.org/r/alpine.LRH.2.02.1607221711410.4818@file01.intranet.prod.int.rdu2.redhat.com
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
1 file changed
