mm: optimize copy_page_to/from_iter_iovec (3fa6c507) · Commits · 戴 / test

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@linux-foundation.org: use IS_ENABLED(), per Andi] 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> Cc: Andi Kleen <andi@firstfloor.org> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

lib/iov_iter.c

+6 −2

Original line number	Diff line number	Diff line
		@@ -144,7 +144,7 @@ static size_t copy_page_to_iter_iovec(struct page *page, size_t offset, size_t b
		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(struct page *page, size_t offset, size_t b
		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(struct page *page, size_t offset, size_t b
		bytes -= copy;
		}
		kunmap(page);

		done:
		if (skip == iov->iov_len) {
		iov++;
		@@ -225,7 +227,7 @@ static size_t copy_page_from_iter_iovec(struct page *page, size_t offset, size_t
		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(struct page *page, size_t offset, size_t
		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(struct page *page, size_t offset, size_t
		bytes -= copy;
		}
		kunmap(page);

		done:
		if (skip == iov->iov_len) {
		iov++;

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