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.kumar(a)linux.ibm.com&gt; --- 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