[PATCH 1/2] block: bio_iov_iter_get_pages: fix size of last iovec

[Martin Wilck]

If the last page of the bio is not "full", the length of the last
vector slot needs to be corrected. This slot has the index
(bio->bi_vcnt - 1), but only in bio->bi_io_vec. In the "bv" helper
array, which is shifted by the value of bio->bi_vcnt at function
invocation, the correct index is (nr_pages - 1).

V2: improved readability following suggestions from Ming Lei.

Fixes: 2cefe4dbaadf ("block: add bio_iov_iter_get_pages()")
Signed-off-by: Martin Wilck <mwi...@suse.com>
---
 block/bio.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 67eff5e..0964328 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -912,16 +912,16 @@ EXPORT_SYMBOL(bio_add_page);
  */
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {
-       unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
+       unsigned short idx, nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
        struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
        struct page **pages = (struct page **)bv;
-       size_t offset, diff;
+       size_t offset;
        ssize_t size;
 
        size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
        if (unlikely(size <= 0))
                return size ? size : -EFAULT;
-       nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
+       idx = nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
 
        /*
         * Deep magic below:  We need to walk the pinned pages backwards
@@ -934,17 +934,15 @@ int bio_iov_iter_get_pages(struct bio *bio, struct 
iov_iter *iter)
        bio->bi_iter.bi_size += size;
        bio->bi_vcnt += nr_pages;
 
-       diff = (nr_pages * PAGE_SIZE - offset) - size;
-       while (nr_pages--) {
-               bv[nr_pages].bv_page = pages[nr_pages];
-               bv[nr_pages].bv_len = PAGE_SIZE;
-               bv[nr_pages].bv_offset = 0;
+       while (idx--) {
+               bv[idx].bv_page = pages[idx];
+               bv[idx].bv_len = PAGE_SIZE;
+               bv[idx].bv_offset = 0;
        }
 
        bv[0].bv_offset += offset;
        bv[0].bv_len -= offset;
-       if (diff)
-               bv[bio->bi_vcnt - 1].bv_len -= diff;
+       bv[nr_pages - 1].bv_len -= nr_pages * PAGE_SIZE - offset - size;
 
        iov_iter_advance(iter, size);
        return 0;
-- 
2.17.1