block: use on-stack page vec for <= UIO_FASTIOV

Avoid a kmalloc+kfree for each page array, if we only have a few pages
that are mapped. An alloc+free for each IO is quite expensive, and
it's pretty pointless if we're only dealing with 1 or a few vecs.

Use UIO_FASTIOV like we do in other spots to set a sane limit for how
big of an IO we want to avoid allocations for.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-map.c b/block/blk-map.c
index 16153fd..f376887 100644 (file)
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -268,12 +268,19 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
        }
 
        while (iov_iter_count(iter)) {
-               struct page **pages;
+               struct page **pages, *stack_pages[UIO_FASTIOV];
                ssize_t bytes;
                size_t offs, added = 0;
                int npages;
 
-               bytes = iov_iter_get_pages_alloc2(iter, &pages, LONG_MAX, &offs);
+               if (nr_vecs <= ARRAY_SIZE(stack_pages)) {
+                       pages = stack_pages;
+                       bytes = iov_iter_get_pages2(iter, pages, LONG_MAX,
+                                                       nr_vecs, &offs);
+               } else {
+                       bytes = iov_iter_get_pages_alloc2(iter, &pages,
+                                                       LONG_MAX, &offs);
+               }
                if (unlikely(bytes <= 0)) {
                        ret = bytes ? bytes : -EFAULT;
                        goto out_unmap;
@@ -309,7 +316,8 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
                 */
                while (j < npages)
                        put_page(pages[j++]);
-               kvfree(pages);
+               if (pages != stack_pages)
+                       kvfree(pages);
                /* couldn't stuff something into bio? */
                if (bytes) {
                        iov_iter_revert(iter, bytes);