[PATCH V1] lib/scatterlist: Fix to calculate the last_pg properly

[Yishai Hadas <yishaih@nvidia.com>]

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: 1567b49d1a40 ("lib/scatterlist: add check when merging zone device pages")
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
---
Changelog:
v1:
 * Rename 'paddr' to 'next_pfn' as it's actually.
 * Move to use page_to_phys() to clarify the code.
 * Fix to use (next_pfn - 1) instead of (paddr - PAGE_SIZE).
v0:
https://lore.kernel.org/lkml/Y7zyyTxdoJulq7OD@casper.infradead.org/T/

 lib/scatterlist.c | 25 +++++++++++++++----------
 1 file changed, 15 insertions(+), 10 deletions(-)

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index f72aa50c6654..8d7519a8f308 100644
--- a/lib/scatterlist.c
+++ b/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 */
-- 
2.18.1