Re: [PATCH v6] mm/pgmap: Use correct alignment when looking at first pfn from a region

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 25 Sep 2019 09:21:02 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:

> Andrew Morton <akpm@linux-foundation.org> writes:
> 
> > On Tue, 17 Sep 2019 21:01:29 +0530 "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> wrote:
> >
> >> 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);
> >
> > What are the user-visible runtime effects of this change?
> 
> This was found by code inspection. If the pmem region is not correctly
> section aligned we can skip pfns while iterating device pfn using 
> 	for_each_device_pfn(pfn, pgmap)
> 
> 
> I still would want Dan to ack the change though.
> 

Dan?


From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Subject: mm/pgmap: use correct alignment when looking at first pfn from a region

vmem_altmap_offset() adjusts 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);

This was found by code inspection. If the pmem region is not correctly
section aligned we can skip pfns while iterating device pfn using 

	for_each_device_pfn(pfn, pgmap)

[akpm@linux-foundation.org: coding style fixes]
Link: http://lkml.kernel.org/r/20190917153129.12905-1-aneesh.kumar@linux.ibm.com
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/memremap.c~mm-pgmap-use-correct-alignment-when-looking-at-first-pfn-from-a-region
+++ a/mm/memremap.c
@@ -55,8 +55,16 @@ static void pgmap_array_delete(struct re
 
 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)
_