Am 06.01.2021 um 05:08 schrieb Dan Williams [off-list ref]:

While pfn_to_online_page() is able to determine pfn_valid() at
subsection granularity it is not able to reliably determine if a given
pfn is also online if the section is mixed with ZONE_DEVICE pfns.

Update move_pfn_range_to_zone() to flag (SECTION_TAINT_ZONE_DEVICE) a
section that mixes ZONE_DEVICE pfns with other online pfns. With
SECTION_TAINT_ZONE_DEVICE to delineate, pfn_to_online_page() can fall
back to a slow-path check for ZONE_DEVICE pfns in an online section.

With this implementation of pfn_to_online_page() pfn-walkers mostly only
need to check section metadata to determine pfn validity. In the
rare case of mixed-zone sections the pfn-walker will skip offline
ZONE_DEVICE pfns as expected.

Other notes:

Because the collision case is rare, and for simplicity, the
SECTION_TAINT_ZONE_DEVICE flag is never cleared once set.

pfn_to_online_page() was already borderline too large to be a macro /
inline function, but the additional logic certainly pushed it over that
threshold, and so it is moved to an out-of-line helper.

Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
Cc: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Michal Hocko <mhocko@suse.com>
Reported-by: David Hildenbrand <redacted>
Signed-off-by: Dan Williams <redacted>
---

This compiles and passes the nvdimm unit tests, but I have not tested
with pfn walkers in the presence of ZONE_DEVICE collisions.


include/linux/memory_hotplug.h |   17 +---------
include/linux/mmzone.h         |   22 ++++++++----
mm/memory_hotplug.c            |   71 ++++++++++++++++++++++++++++++++++++++++
3 files changed, 87 insertions(+), 23 deletions(-)

diff --git a/include/linux/memory_hotplug.h b/include/linux/memory_hotplug.h
index 15acce5ab106..3d99de0db2dd 100644
--- a/include/linux/memory_hotplug.h
+++ b/include/linux/memory_hotplug.h

@@ -16,22 +16,7 @@ struct resource;

struct vmem_altmap;

#ifdef CONFIG_MEMORY_HOTPLUG
-/*
- * Return page for the valid pfn only if the page is online. All pfn
- * walkers which rely on the fully initialized page->flags and others
- * should use this rather than pfn_valid && pfn_to_page
- */
-#define pfn_to_online_page(pfn)                       \
-({                                   \
-    struct page *___page = NULL;                   \
-    unsigned long ___pfn = pfn;                   \
-    unsigned long ___nr = pfn_to_section_nr(___pfn);       \
-                                   \
-    if (___nr < NR_MEM_SECTIONS && online_section_nr(___nr) && \
-        pfn_valid_within(___pfn))                   \
-        ___page = pfn_to_page(___pfn);               \
-    ___page;                           \
-})
+struct page *pfn_to_online_page(unsigned long pfn);

/*
 * Types for free bootmem stored in page->lru.next. These have to be in

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index b593316bff3d..0b5c44f730b4 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h

@@ -1273,13 +1273,14 @@ extern size_t mem_section_usage_size(void);
 *      which results in PFN_SECTION_SHIFT equal 6.
 * To sum it up, at least 6 bits are available.
 */
-#define    SECTION_MARKED_PRESENT    (1UL<<0)
-#define SECTION_HAS_MEM_MAP    (1UL<<1)
-#define SECTION_IS_ONLINE    (1UL<<2)
-#define SECTION_IS_EARLY    (1UL<<3)
-#define SECTION_MAP_LAST_BIT    (1UL<<4)
-#define SECTION_MAP_MASK    (~(SECTION_MAP_LAST_BIT-1))
-#define SECTION_NID_SHIFT    3
+#define SECTION_MARKED_PRESENT        (1UL<<0)
+#define SECTION_HAS_MEM_MAP        (1UL<<1)
+#define SECTION_IS_ONLINE        (1UL<<2)
+#define SECTION_IS_EARLY        (1UL<<3)
+#define SECTION_TAINT_ZONE_DEVICE    (1UL<<4)
+#define SECTION_MAP_LAST_BIT        (1UL<<5)
+#define SECTION_MAP_MASK        (~(SECTION_MAP_LAST_BIT-1))
+#define SECTION_NID_SHIFT        3

static inline struct page *__section_mem_map_addr(struct mem_section *section)
{

@@ -1318,6 +1319,13 @@ static inline int online_section(struct mem_section *section)
   return (section && (section->section_mem_map & SECTION_IS_ONLINE));

}

+static inline int online_device_section(struct mem_section *section)
+{
+    unsigned long flags = SECTION_IS_ONLINE | SECTION_TAINT_ZONE_DEVICE;
+
+    return section && ((section->section_mem_map & flags) == flags);
+}
+
static inline int online_section_nr(unsigned long nr)
{
   return online_section(__nr_to_section(nr));

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index f9d57b9be8c7..9f36968e6188 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c

@@ -300,6 +300,47 @@ static int check_hotplug_memory_addressable(unsigned long pfn,
   return 0;

}

+/*
+ * Return page for the valid pfn only if the page is online. All pfn
+ * walkers which rely on the fully initialized page->flags and others
+ * should use this rather than pfn_valid && pfn_to_page
+ */
+struct page *pfn_to_online_page(unsigned long pfn)
+{
+    unsigned long nr = pfn_to_section_nr(pfn);
+    struct dev_pagemap *pgmap;
+    struct mem_section *ms;
+
+    if (nr >= NR_MEM_SECTIONS)
+        return NULL;
+
+    ms = __nr_to_section(nr);
+
+    if (!online_section(ms))
+        return NULL;
+
+    if (!pfn_valid_within(pfn))
+        return NULL;
+
+    if (!online_device_section(ms))
+        return pfn_to_page(pfn);
+
+    /*
+     * Slowpath: when ZONE_DEVICE collides with
+     * ZONE_{NORMAL,MOVABLE} within the same section some pfns in
+     * the section may be 'offline' but 'valid'. Only
+     * get_dev_pagemap() can determine sub-section online status.
+     */
+    pgmap = get_dev_pagemap(pfn, NULL);
+    put_dev_pagemap(pgmap);
+
+    /* The presence of a pgmap indicates ZONE_DEVICE offline pfn */
+    if (pgmap)
+        return NULL;
+    return pfn_to_page(pfn);
+}
+EXPORT_SYMBOL_GPL(pfn_to_online_page);
+
/*
 * Reasonably generic function for adding memory.  It is
 * expected that archs that support memory hotplug will

@@ -678,6 +719,27 @@ static void __meminit resize_pgdat_range(struct pglist_data *pgdat, unsigned lon
   pgdat->node_spanned_pages = max(start_pfn + nr_pages, old_end_pfn) - pgdat->node_start_pfn;

}
+
+static int zone_id(const struct zone *zone)
+{
+    struct pglist_data *pgdat = zone->zone_pgdat;
+
+    return zone - pgdat->node_zones;
+}
+
+static void section_taint_zone_device(struct zone *zone, unsigned long pfn)
+{
+    struct mem_section *ms = __nr_to_section(pfn_to_section_nr(pfn));
+
+    if (zone_id(zone) != ZONE_DEVICE)
+        return;
+
+    if (IS_ALIGNED(pfn, PAGES_PER_SECTION))
+        return;