[PATCH] iommufd: Fix offset handling in folio subroutines

[Alexey Kardashevskiy <aik@amd.com>]

The @offset from memfd_pin_folios() is pgoff_t and is bytes, not
a number of pages as batch_from_folios() treats it.

Convert byte offset to pfn offset and use that.

Fixes: ed9178fbfd4e ("iommufd: Folio subroutines")
Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
---

Found this when tried mapping a bunch of folios (>64) with an offset and
the first folio was used for all the mappings in the batch.

---
 drivers/iommu/iommufd/pages.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/iommufd/pages.c b/drivers/iommu/iommufd/pages.c
index 3427749bc5ce..6075d11691a6 100644
--- a/drivers/iommu/iommufd/pages.c
+++ b/drivers/iommu/iommufd/pages.c
@@ -646,8 +646,9 @@ static int batch_from_folios(struct pfn_batch *batch, struct folio ***folios_p,
 
 	while (npages) {
 		struct folio *folio = *folios;
-		unsigned long nr = folio_nr_pages(folio) - offset;
-		unsigned long pfn = page_to_pfn(folio_page(folio, offset));
+		unsigned long offset_nr = offset >> PAGE_SHIFT;
+		unsigned long nr = folio_nr_pages(folio) - offset_nr;
+		unsigned long pfn = page_to_pfn(folio_page(folio, offset_nr));
 
 		nr = min(nr, npages);
 		npages -= nr;
-- 
2.47.1