[PATCH 32/46] mm/memory_hotplug: avoid races in pfn_to_online_page() with dying pgmap

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Josef Bacik <josef@toxicpanda.com>

If ZONE_DEVICE memory shares a memory section with ordinary memory, the
section is online and the pfns of the device are valid.
pfn_to_online_page() tells them apart by asking get_dev_pagemap() whether
the pfn belongs to a dev_pagemap.

get_dev_pagemap() only returns a dev_pagemap whose reference is alive.
memunmap_pages() kills the reference first, waits until all pages are
idle, and then removes the memmap. During all that time the function
finds nothing, and pfn_to_online_page() hands out the struct pages of the
device as if they were online memory. A pfn walker reads them while they
are poisoned and freed.

The entry in pgmap_array is only deleted after the pages have been
removed. Look at that instead of trying to get a reference.

This is probably not worth CCing stable.

Fix generated by an LLM and sent by Josef Bacik in private.

Fixes: 1f90a3477df3 ("mm: teach pfn_to_online_page() about ZONE_DEVICE section collisions")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 include/linux/memremap.h |  6 ++++++
 mm/memory_hotplug.c      | 13 +++++--------
 mm/memremap.c            | 13 +++++++++++++
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index e3c2ccf872a8a..b5e5e626c7a59 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -231,6 +231,7 @@ void memunmap_pages(struct dev_pagemap *pgmap);
 void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
 void devm_memunmap_pages(struct device *dev, struct dev_pagemap *pgmap);
 struct dev_pagemap *get_dev_pagemap(unsigned long pfn);
+bool pfn_has_dev_pagemap(unsigned long pfn);
 bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
 
 unsigned long memremap_compat_align(void);
@@ -284,6 +285,11 @@ static inline struct dev_pagemap *get_dev_pagemap(unsigned long pfn)
 	return NULL;
 }
 
+static inline bool pfn_has_dev_pagemap(unsigned long pfn)
+{
+	return false;
+}
+
 static inline bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn)
 {
 	return false;
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 796af1028ee23..5a972d7a9e478 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -342,7 +342,6 @@ static int check_pfn_span(unsigned long pfn, unsigned long nr_pages)
 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;
 
 	BUILD_BUG_ON(IS_ENABLED(CONFIG_HAVE_ARCH_PFN_VALID));
@@ -363,14 +362,12 @@ struct page *pfn_to_online_page(unsigned long 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.
+	 * the section may be 'offline' but 'valid'. Only the presence of a
+	 * pgmap can determine sub-section online status: it indicates a
+	 * ZONE_DEVICE offline pfn. That includes a pgmap whose reference has
+	 * been killed, its memmap is about to go away.
 	 */
-	pgmap = get_dev_pagemap(pfn);
-	put_dev_pagemap(pgmap);
-
-	/* The presence of a pgmap indicates ZONE_DEVICE offline pfn */
-	if (pgmap)
+	if (pfn_has_dev_pagemap(pfn))
 		return NULL;
 
 	return pfn_to_page(pfn);
diff --git a/mm/memremap.c b/mm/memremap.c
index accba23aef28c..a1e99a4c55c56 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -413,6 +413,19 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn)
 }
 EXPORT_SYMBOL_GPL(get_dev_pagemap);
 
+/**
+ * pfn_has_dev_pagemap() - does @pfn belong to a dev_pagemap, dead or alive
+ * @pfn: page frame number to lookup page_map
+ *
+ * In contrast to get_dev_pagemap() this also finds a dev_pagemap which is on
+ * its way out: it is registered before its pages are added, and until they
+ * have been removed.
+ */
+bool pfn_has_dev_pagemap(unsigned long pfn)
+{
+	return xa_load(&pgmap_array, pfn);
+}
+
 void free_zone_device_folio(struct folio *folio)
 {
 	struct dev_pagemap *pgmap = folio->pgmap;

-- 
2.43.0