From: Mikulas Patocka <mpatocka@xxxxxxxxxx> Subject: 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 [akpm@xxxxxxxxxxxxxxxxxxxx: use IS_ENABLED(), per Andi] Link: http://lkml.kernel.org/r/alpine.LRH.2.02.1607221711410.4818@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/iov_iter.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff -puN lib/iov_iter.c~mm-optimize-copy_page_to-from_iter_iovec lib/iov_iter.c --- a/lib/iov_iter.c~mm-optimize-copy_page_to-from_iter_iovec +++ a/lib/iov_iter.c @@ -144,7 +144,7 @@ static size_t copy_page_to_iter_iovec(st buf = iov->iov_base + skip; copy = min(bytes, iov->iov_len - skip); - if (!fault_in_pages_writeable(buf, copy)) { + if (IS_ENABLED(CONFIG_HIGHMEM) && !fault_in_pages_writeable(buf, copy)) { kaddr = kmap_atomic(page); from = kaddr + offset; @@ -175,6 +175,7 @@ static size_t copy_page_to_iter_iovec(st copy = min(bytes, iov->iov_len - skip); } /* Too bad - revert to non-atomic kmap */ + kaddr = kmap(page); from = kaddr + offset; left = __copy_to_user(buf, from, copy); @@ -193,6 +194,7 @@ static size_t copy_page_to_iter_iovec(st bytes -= copy; } kunmap(page); + done: if (skip == iov->iov_len) { iov++; @@ -225,7 +227,7 @@ static size_t copy_page_from_iter_iovec( buf = iov->iov_base + skip; copy = min(bytes, iov->iov_len - skip); - if (!fault_in_pages_readable(buf, copy)) { + if (IS_ENABLED(CONFIG_HIGHMEM) && !fault_in_pages_readable(buf, copy)) { kaddr = kmap_atomic(page); to = kaddr + offset; @@ -256,6 +258,7 @@ static size_t copy_page_from_iter_iovec( copy = min(bytes, iov->iov_len - skip); } /* Too bad - revert to non-atomic kmap */ + kaddr = kmap(page); to = kaddr + offset; left = __copy_from_user(to, buf, copy); @@ -274,6 +277,7 @@ static size_t copy_page_from_iter_iovec( bytes -= copy; } kunmap(page); + done: if (skip == iov->iov_len) { iov++; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
