vmem_altmap_offset() adjust the section aligned base_pfn offset.
So we need to make sure we account for the same when computing base_pfn.

ie, for altmap_valid case, our pfn_first should be:

pfn_first = altmap->base_pfn + vmem_altmap_offset(altmap);

Signed-off-by: Aneesh Kumar K.V <aneesh.ku...@linux.ibm.com>
---
Changes from v4:
* rebase to latest kernel

 mm/memremap.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/memremap.c b/mm/memremap.c
index 6ee03a816d67..6b8cd10e5e35 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -54,8 +54,16 @@ static void pgmap_array_delete(struct resource *res)
 
 static unsigned long pfn_first(struct dev_pagemap *pgmap)
 {
-       return PHYS_PFN(pgmap->res.start) +
-               vmem_altmap_offset(pgmap_altmap(pgmap));
+       const struct resource *res = &pgmap->res;
+       struct vmem_altmap *altmap = pgmap_altmap(pgmap);
+       unsigned long pfn;
+
+       if (altmap) {
+               pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
+       } else
+               pfn = PHYS_PFN(res->start);
+
+       return pfn;
 }
 
 static unsigned long pfn_end(struct dev_pagemap *pgmap)
-- 
2.21.0

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