On Wed, Nov 21, 2018 at 11:23:23AM +0800, Ming Lei wrote:
>       if (bio->bi_vcnt > 0) {
> -             struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
> +             struct bio_vec bv;
> +             struct bio_vec *seg = &bio->bi_io_vec[bio->bi_vcnt - 1];
>  
> -             if (page == bv->bv_page && off == bv->bv_offset + bv->bv_len) {
> -                     bv->bv_len += len;
> +             bvec_last_segment(seg, &bv);
> +
> +             if (page == bv.bv_page && off == bv.bv_offset + bv.bv_len) {

I think this we can simplify the try to merge into bio case a bit,
and also document it better with something like this:

diff --git a/block/bio.c b/block/bio.c
index 854676edc438..cc913281a723 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -822,54 +822,40 @@ EXPORT_SYMBOL(bio_add_pc_page);
  * @page: page to add
  * @len: length of the data to add
  * @off: offset of the data in @page
+ * @same_page: if %true only merge if the new data is in the same physical
+ *             page as the last segment of the bio.
  *
- * Try to add the data at @page + @off to the last page of @bio.  This is a
+ * Try to add the data at @page + @off to the last bvec of @bio.  This is a
  * a useful optimisation for file systems with a block size smaller than the
  * page size.
  *
  * Return %true on success or %false on failure.
  */
 bool __bio_try_merge_page(struct bio *bio, struct page *page,
-               unsigned int len, unsigned int off)
+               unsigned int len, unsigned int off, bool same_page)
 {
        if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
                return false;
 
        if (bio->bi_vcnt > 0) {
-               struct bio_vec bv;
-               struct bio_vec *seg = &bio->bi_io_vec[bio->bi_vcnt - 1];
-
-               bvec_last_segment(seg, &bv);
-
-               if (page == bv.bv_page && off == bv.bv_offset + bv.bv_len) {
-                       seg->bv_len += len;
-                       bio->bi_iter.bi_size += len;
-                       return true;
-               }
+               struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
+               phys_addr_t vec_addr = page_to_phys(bv->bv_page);
+               phys_addr_t page_addr = page_to_phys(page);
+
+               if (vec_addr + bv->bv_offset + bv->bv_len != page_addr + off)
+                       return false;
+               if (same_page &&
+                   (vec_addr & PAGE_SIZE) != (page_addr & PAGE_SIZE))
+                       return false;
+
+               bv->bv_len += len;
+               bio->bi_iter.bi_size += len;
+               return true;
        }
        return false;
 }
 EXPORT_SYMBOL_GPL(__bio_try_merge_page);
 
-static bool bio_try_merge_segment(struct bio *bio, struct page *page,
-                                 unsigned int len, unsigned int off)
-{
-       if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
-               return false;
-
-       if (bio->bi_vcnt > 0) {
-               struct bio_vec *seg = &bio->bi_io_vec[bio->bi_vcnt - 1];
-
-               if (page_to_phys(seg->bv_page) + seg->bv_offset + seg->bv_len ==
-                   page_to_phys(page) + off) {
-                       seg->bv_len += len;
-                       bio->bi_iter.bi_size += len;
-                       return true;
-               }
-       }
-       return false;
-}
-
 /**
  * __bio_add_page - add page to a bio in a new segment
  * @bio: destination bio
@@ -910,7 +896,7 @@ EXPORT_SYMBOL_GPL(__bio_add_page);
 int bio_add_page(struct bio *bio, struct page *page,
                 unsigned int len, unsigned int offset)
 {
-       if (!bio_try_merge_segment(bio, page, len, offset)) {
+       if (!__bio_try_merge_page(bio, page, len, offset, false)) {
                if (bio_full(bio))
                        return 0;
                __bio_add_page(bio, page, len, offset);
diff --git a/fs/iomap.c b/fs/iomap.c
index ccc2ba115f4d..d918acb9bfc9 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -313,7 +313,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, 
loff_t length, void *data,
         */
        sector = iomap_sector(iomap, pos);
        if (ctx->bio && bio_end_sector(ctx->bio) == sector) {
-               if (__bio_try_merge_page(ctx->bio, page, plen, poff))
+               if (__bio_try_merge_page(ctx->bio, page, plen, poff, true))
                        goto done;
                is_contig = true;
        }
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 5c2190216614..b9fd44168f61 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -616,7 +616,7 @@ xfs_add_to_ioend(
                                bdev, sector);
        }
 
-       if (!__bio_try_merge_page(wpc->ioend->io_bio, page, len, poff)) {
+       if (!__bio_try_merge_page(wpc->ioend->io_bio, page, len, poff, true)) {
                if (iop)
                        atomic_inc(&iop->write_count);
                if (bio_full(wpc->ioend->io_bio))
diff --git a/include/linux/bio.h b/include/linux/bio.h
index e5b975fa0558..f08e6940c1ab 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -442,7 +442,7 @@ extern int bio_add_page(struct bio *, struct page *, 
unsigned int,unsigned int);
 extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
                           unsigned int, unsigned int);
 bool __bio_try_merge_page(struct bio *bio, struct page *page,
-               unsigned int len, unsigned int off);
+               unsigned int len, unsigned int off, bool same_page);
 void __bio_add_page(struct bio *bio, struct page *page,
                unsigned int len, unsigned int off);
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);

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