block: Fix page_is_mergeable() for compound pages

If we pass in an offset which is larger than PAGE_SIZE, then
page_is_mergeable() thinks it's not mergeable with the previous bio_vec,
leading to a large number of bio_vecs being used.  Use a slightly more
obvious test that the two pages are compatible with each other.

Fixes: 52d52d1c ("block: only allow contiguous page structs in a bio_vec")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -740,8 +740,8 @@ static inline bool page_is_mergeable(const struct bio_vec *bv,
 		struct page *page, unsigned int len, unsigned int off,
 		bool *same_page)
 {
-	phys_addr_t vec_end_addr = page_to_phys(bv->bv_page) +
-		bv->bv_offset + bv->bv_len - 1;
+	size_t bv_end = bv->bv_offset + bv->bv_len;
+	phys_addr_t vec_end_addr = page_to_phys(bv->bv_page) + bv_end - 1;
 	phys_addr_t page_addr = page_to_phys(page);
 
 	if (vec_end_addr + 1 != page_addr + off)
@@ -750,9 +750,9 @@ static inline bool page_is_mergeable(const struct bio_vec *bv,
 		return false;
 
 	*same_page = ((vec_end_addr & PAGE_MASK) == page_addr);
-	if (!*same_page && pfn_to_page(PFN_DOWN(vec_end_addr)) + 1 != page)
-		return false;
+	if (*same_page)
 		return true;
+	return (bv->bv_page + bv_end / PAGE_SIZE) == (page + off / PAGE_SIZE);
 }
 
 /*