From: Dan Williams <redacted>
Subject: mm: teach pfn_to_online_page() to consider subsection validity

pfn_to_online_page is primarily used to filter out offline or fully
uninitialized pages.  pfn_valid resp.  online_section_nr have a coarse
per memory section granularity.  If a section shared with a partially
offline memory (e.g.  part of ZONE_DEVICE) then pfn_to_online_page
would lead to a false positive on some pfns.  Fix this by adding
pfn_section_valid check which is subsection aware.

[mhocko@kernel.org: changelog rewrite]
Link: https://lkml.kernel.org/r/161058500148.1840162.4365921007820501696.stgit@dwillia2-desk3.amr.corp.intel.com
Fixes: b13bc35193d9 ("mm/hotplug: invalid PFNs from pfn_to_online_page()")
Signed-off-by: Dan Williams <redacted>
Reported-by: David Hildenbrand <redacted>
Reviewed-by: David Hildenbrand <redacted>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Qian Cai <redacted>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Naoya Horiguchi <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/memory_hotplug.c |   23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

--- a/mm/memory_hotplug.c~mm-teach-pfn_to_online_page-to-consider-subsection-validity
+++ a/mm/memory_hotplug.c

@@ -308,11 +308,26 @@ static int check_hotplug_memory_addressa
 struct page *pfn_to_online_page(unsigned long pfn)
 {
 	unsigned long nr = pfn_to_section_nr(pfn);
+	struct mem_section *ms;
 
-	if (nr < NR_MEM_SECTIONS && online_section_nr(nr) &&
-	    pfn_valid_within(pfn))
-		return pfn_to_page(pfn);
-	return NULL;
+	if (nr >= NR_MEM_SECTIONS)
+		return NULL;
+
+	ms = __nr_to_section(nr);
+	if (!online_section(ms))
+		return NULL;
+
+	/*
+	 * Save some code text when online_section() +
+	 * pfn_section_valid() are sufficient.
+	 */
+	if (IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID) && !pfn_valid(pfn))
+		return NULL;
+
+	if (!pfn_section_valid(ms, pfn))
+		return NULL;
+
+	return pfn_to_page(pfn);
 }
 EXPORT_SYMBOL_GPL(pfn_to_online_page);
 

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