lib/scatterlist: Fix to calculate the last_pg properly

The last_pg is wrong, it is actually the first page of the last
scatterlist element. To get the last page of the last scatterlist element
we have to add prv->length. So it is checking mergability against the
wrong page, Further, a SG element is not guaranteed to end on a page
boundary, so we have to check the sub page location also for merge
eligibility.

Fix the above by checking physical contiguity based on PFNs, compute the
actual last page and then call pages_are_mergable().

Fixes: 1567b49d ("lib/scatterlist: add check when merging zone device pages")
Link: https://lore.kernel.org/r/20230111101054.188136-1-yishaih@nvidia.comReported-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

lib/scatterlist.c

@@ -470,22 +470,27 @@ int sg_alloc_append_table_from_pages(struct sg_append_table *sgt_append,
 		return -EOPNOTSUPP;
 
 	if (sgt_append->prv) {
+		unsigned long next_pfn = (page_to_phys(sg_page(sgt_append->prv)) +
+			sgt_append->prv->offset + sgt_append->prv->length) / PAGE_SIZE;
+
 		if (WARN_ON(offset))
 			return -EINVAL;
 
 		/* Merge contiguous pages into the last SG */
 		prv_len = sgt_append->prv->length;
-		last_pg = sg_page(sgt_append->prv);
-		while (n_pages && pages_are_mergeable(pages[0], last_pg)) {
-			if (sgt_append->prv->length + PAGE_SIZE > max_segment)
-				break;
-			sgt_append->prv->length += PAGE_SIZE;
-			last_pg = pages[0];
-			pages++;
-			n_pages--;
+		if (page_to_pfn(pages[0]) == next_pfn) {
+			last_pg = pfn_to_page(next_pfn - 1);
+			while (n_pages && pages_are_mergeable(pages[0], last_pg)) {
+				if (sgt_append->prv->length + PAGE_SIZE > max_segment)
+					break;
+				sgt_append->prv->length += PAGE_SIZE;
+				last_pg = pages[0];
+				pages++;
+				n_pages--;
+			}
+			if (!n_pages)
+				goto out;
 		}
-		if (!n_pages)
-			goto out;
 	}
 
 	/* compute number of contiguous chunks */