[PATCH v3 0/3] mm: page_ext: Introduce new iteration API

[PATCH v3 0/3] mm: page_ext: Introduce new iteration API

[Luiz Capitulino]

This series is against v6.14-rc5. The introduction is after the changelog.

v2 -> v3
========

- Change the new API to take a PFN number instead of a struct page, which
  solves header file issue (David)
- Optimize page_ext_get()/page_ext_put() in __folio_copy_owner (David)

v1 -> v2
========

- Drop for_each_page_ext_order() (David)
- Make page_ext_iter_begin() and page_ext_iter_next() inline functions
  (David)
- Move loop logic into page_ext_iter_begin() and page_ext_iter_next()
  (David)

RFC -> v1
=========

- Revamped the API by introducing for_each_page_ext macros
- Implemented various suggestions from David Hildenbrand, including page_ext
  lookup optimization
- Fixed changelogs

Introduction
============

  [ Thanks to David Hildenbrand for identifying the root cause of this
    issue and proving guidance on how to fix it. The new API idea, bugs
    and misconceptions are all mine though ]

Currently, trying to reserve 1G pages with page_owner=on and sparsemem
causes a crash. The reproducer is very simple:

 1. Build the kernel with CONFIG_SPARSEMEM=y and the table extensions
 2. Pass 'default_hugepagesz=1 page_owner=on' in the kernel command-line
 3. Reserve one 1G page at run-time, this should crash (see patch 1 for
    backtrace) 

 [ A crash with page_table_check is also possible, but harder to trigger ]

Apparently, starting with commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP
for gigantic folios") we now pass the full allocation order to page
extension clients and the page extension implementation assumes that all
PFNs of an allocation range will be stored in the same memory section (which
is not true for 1G pages).

To fix this, this series introduces a new iteration API for page extension
objects. The API checks if the next page extension object can be retrieved
from the current section or if it needs to look up for it in another
section.

Please, find all details in patch 1.

I tested this series on arm64 and x86 by reserving 1G pages at run-time
and doing kernel builds (always with page_owner=on and page_table_check=on).

Luiz Capitulino (3):
  mm: page_ext: add an iteration API for page extensions
  mm: page_table_check: use new iteration API
  mm: page_owner: use new iteration API

 include/linux/page_ext.h | 93 ++++++++++++++++++++++++++++++++++++++++
 mm/page_ext.c            | 13 ++++++
 mm/page_owner.c          | 84 +++++++++++++++++-------------------
 mm/page_table_check.c    | 39 ++++++-----------
 4 files changed, 157 insertions(+), 72 deletions(-)

-- 
2.48.1

[PATCH v3 1/3] mm: page_ext: add an iteration API for page extensions

[Luiz Capitulino]

The page extension implementation assumes that all page extensions of
a given page order are stored in the same memory section. The function
page_ext_next() relies on this assumption by adding an offset to the
current object to return the next adjacent page extension.

This behavior works as expected for flatmem but fails for sparsemem when
using 1G pages. The commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for
gigantic folios") exposes this issue, making it possible for a crash when
using page_owner or page_table_check page extensions.

The problem is that for 1G pages, the page extensions may span memory
section boundaries and be stored in different memory sections. This issue
was not visible before commit cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP
for gigantic folios") because alloc_contig_pages() never passed more than
MAX_PAGE_ORDER to post_alloc_hook(). However, the series introducing
mentioned commit changed this behavior allowing the full 1G page order
to be passed.

Reproducer:

 1. Build the kernel with CONFIG_SPARSEMEM=y and table extensions
    support
 2. Pass 'default_hugepagesz=1 page_owner=on' in the kernel command-line
 3. Reserve one 1G page at run-time, this should crash (backtrace below)

To address this issue, this commit introduces a new API for iterating
through page extensions. The main iteration macro is for_each_page_ext()
and it must be called with the RCU read lock taken. Here's an usage
example:

"""
struct page_ext_iter iter;
struct page_ext *page_ext;

...

rcu_read_lock();
for_each_page_ext(page, 1 << order, page_ext, iter) {
	struct my_page_ext *obj = get_my_page_ext_obj(page_ext);
	...
}
rcu_read_unlock();
"""

The loop construct uses page_ext_iter_next() which checks to see if we
have crossed sections in the iteration. In this case, page_ext_iter_next()
retrieves the next page_ext object from another section.

Thanks to David Hildenbrand for helping identify the root cause and
providing suggestions on how to fix and optmize the solution (final
implementation and bugs are all mine through).

Lastly, here's the backtrace, without kasan you can get random crashes:

[   76.052526] BUG: KASAN: slab-out-of-bounds in __update_page_owner_handle+0x238/0x298
[   76.060283] Write of size 4 at addr ffff07ff96240038 by task tee/3598
[   76.066714]
[   76.068203] CPU: 88 UID: 0 PID: 3598 Comm: tee Kdump: loaded Not tainted 6.13.0-rep1 #3
[   76.076202] Hardware name: WIWYNN Mt.Jade Server System B81.030Z1.0007/Mt.Jade Motherboard, BIOS 2.10.20220810 (SCP: 2.10.20220810) 2022/08/10
[   76.088972] Call trace:
[   76.091411]  show_stack+0x20/0x38 (C)
[   76.095073]  dump_stack_lvl+0x80/0xf8
[   76.098733]  print_address_description.constprop.0+0x88/0x398
[   76.104476]  print_report+0xa8/0x278
[   76.108041]  kasan_report+0xa8/0xf8
[   76.111520]  __asan_report_store4_noabort+0x20/0x30
[   76.116391]  __update_page_owner_handle+0x238/0x298
[   76.121259]  __set_page_owner+0xdc/0x140
[   76.125173]  post_alloc_hook+0x190/0x1d8
[   76.129090]  alloc_contig_range_noprof+0x54c/0x890
[   76.133874]  alloc_contig_pages_noprof+0x35c/0x4a8
[   76.138656]  alloc_gigantic_folio.isra.0+0x2c0/0x368
[   76.143616]  only_alloc_fresh_hugetlb_folio.isra.0+0x24/0x150
[   76.149353]  alloc_pool_huge_folio+0x11c/0x1f8
[   76.153787]  set_max_huge_pages+0x364/0xca8
[   76.157961]  __nr_hugepages_store_common+0xb0/0x1a0
[   76.162829]  nr_hugepages_store+0x108/0x118
[   76.167003]  kobj_attr_store+0x3c/0x70
[   76.170745]  sysfs_kf_write+0xfc/0x188
[   76.174492]  kernfs_fop_write_iter+0x274/0x3e0
[   76.178927]  vfs_write+0x64c/0x8e0
[   76.182323]  ksys_write+0xf8/0x1f0
[   76.185716]  __arm64_sys_write+0x74/0xb0
[   76.189630]  invoke_syscall.constprop.0+0xd8/0x1e0
[   76.194412]  do_el0_svc+0x164/0x1e0
[   76.197891]  el0_svc+0x40/0xe0
[   76.200939]  el0t_64_sync_handler+0x144/0x168
[   76.205287]  el0t_64_sync+0x1ac/0x1b0

Fixes: cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 include/linux/page_ext.h | 93 ++++++++++++++++++++++++++++++++++++++++
 mm/page_ext.c            | 13 ++++++
 2 files changed, 106 insertions(+)

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index e4b48a0dda244..76c817162d2fb 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -3,6 +3,7 @@
 #define __LINUX_PAGE_EXT_H
 
 #include <linux/types.h>
+#include <linux/mmzone.h>
 #include <linux/stacktrace.h>
 
 struct pglist_data;
@@ -69,16 +70,31 @@ extern void page_ext_init(void);
 static inline void page_ext_init_flatmem_late(void)
 {
 }
+
+static inline bool page_ext_iter_next_fast_possible(unsigned long next_pfn)
+{
+	/*
+	 * page_ext is allocated per memory section. Once we cross a
+	 * memory section, we have to fetch the new pointer.
+	 */
+	return next_pfn % PAGES_PER_SECTION;
+}
 #else
 extern void page_ext_init_flatmem(void);
 extern void page_ext_init_flatmem_late(void);
 static inline void page_ext_init(void)
 {
 }
+
+static inline bool page_ext_iter_next_fast_possible(unsigned long next_pfn)
+{
+	return true;
+}
 #endif
 
 extern struct page_ext *page_ext_get(const struct page *page);
 extern void page_ext_put(struct page_ext *page_ext);
+extern struct page_ext *page_ext_lookup(unsigned long pfn);
 
 static inline void *page_ext_data(struct page_ext *page_ext,
 				  struct page_ext_operations *ops)
@@ -93,6 +109,83 @@ static inline struct page_ext *page_ext_next(struct page_ext *curr)
 	return next;
 }
 
+struct page_ext_iter {
+	unsigned long index;
+	unsigned long start_pfn;
+	struct page_ext *page_ext;
+};
+
+/**
+ * page_ext_iter_begin() - Prepare for iterating through page extensions.
+ * @iter: page extension iterator.
+ * @pfn: PFN of the page we're interested in.
+ *
+ * Must be called with RCU read lock taken.
+ *
+ * Return: NULL if no page_ext exists for this page.
+ */
+static inline struct page_ext *page_ext_iter_begin(struct page_ext_iter *iter,
+						unsigned long pfn)
+{
+	iter->index = 0;
+	iter->start_pfn = pfn;
+	iter->page_ext = page_ext_lookup(pfn);
+
+	return iter->page_ext;
+}
+
+/**
+ * page_ext_iter_next() - Get next page extension
+ * @iter: page extension iterator.
+ *
+ * Must be called with RCU read lock taken.
+ *
+ * Return: NULL if no next page_ext exists.
+ */
+static inline struct page_ext *page_ext_iter_next(struct page_ext_iter *iter)
+{
+	unsigned long pfn;
+
+	if (WARN_ON_ONCE(!iter->page_ext))
+		return NULL;
+
+	iter->index++;
+	pfn = iter->start_pfn + iter->index;
+
+	if (page_ext_iter_next_fast_possible(pfn))
+		iter->page_ext = page_ext_next(iter->page_ext);
+	else
+		iter->page_ext = page_ext_lookup(pfn);
+
+	return iter->page_ext;
+}
+
+/**
+ * page_ext_iter_get() - Get current page extension
+ * @iter: page extension iterator.
+ *
+ * Return: NULL if no page_ext exists for this iterator.
+ */
+static inline struct page_ext *page_ext_iter_get(const struct page_ext_iter *iter)
+{
+	return iter->page_ext;
+}
+
+/**
+ * for_each_page_ext(): iterate through page_ext objects.
+ * @__page: the page we're interested in
+ * @__pgcount: how many pages to iterate through
+ * @__page_ext: struct page_ext pointer where the current page_ext
+ *              object is returned
+ * @__iter: struct page_ext_iter object (defined in the stack)
+ *
+ * IMPORTANT: must be called with RCU read lock taken.
+ */
+#define for_each_page_ext(__page, __pgcount, __page_ext, __iter) \
+	for (__page_ext = page_ext_iter_begin(&__iter, page_to_pfn(__page));\
+		__page_ext && __iter.index < __pgcount;          \
+		__page_ext = page_ext_iter_next(&__iter))
+
 #else /* !CONFIG_PAGE_EXTENSION */
 struct page_ext;
 
diff --git a/mm/page_ext.c b/mm/page_ext.c
index 641d93f6af4c1..c351fdfe9e9a5 100644
--- a/mm/page_ext.c
+++ b/mm/page_ext.c
@@ -507,6 +507,19 @@ void __meminit pgdat_page_ext_init(struct pglist_data *pgdat)
 
 #endif
 
+/**
+ * page_ext_lookup() - Lookup a page extension for a PFN.
+ * @pfn: PFN of the page we're interested in.
+ *
+ * Must be called with RCU read lock taken and @pfn must be valid.
+ *
+ * Return: NULL if no page_ext exists for this page.
+ */
+struct page_ext *page_ext_lookup(unsigned long pfn)
+{
+	return lookup_page_ext(pfn_to_page(pfn));
+}
+
 /**
  * page_ext_get() - Get the extended information for a page.
  * @page: The page we're interested in.
-- 
2.48.1

Re: [PATCH v3 1/3] mm: page_ext: add an iteration API for page extensions

[David Hildenbrand]

> +++ b/mm/page_ext.c
> @@ -507,6 +507,19 @@ void __meminit pgdat_page_ext_init(struct pglist_data *pgdat)
>   
>   #endif
>   
> +/**
> + * page_ext_lookup() - Lookup a page extension for a PFN.
> + * @pfn: PFN of the page we're interested in.
> + *
> + * Must be called with RCU read lock taken and @pfn must be valid.
> + *
> + * Return: NULL if no page_ext exists for this page.
> + */
> +struct page_ext *page_ext_lookup(unsigned long pfn)
> +{
> +	return lookup_page_ext(pfn_to_page(pfn));
> +}
> +
>   /**
>    * page_ext_get() - Get the extended information for a page.
>    * @page: The page we're interested in.

As discussed offline, getting rid of lookup_page_ext() can be done later 
as a cleanup.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb

[PATCH v3 2/3] mm: page_table_check: use new iteration API

[Luiz Capitulino]

The page_ext_next() function assumes that page extension objects for a
page order allocation always reside in the same memory section, which
may not be true and could lead to crashes. Use the new page_ext
iteration API instead.

Fixes: cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 mm/page_table_check.c | 39 ++++++++++++---------------------------
 1 file changed, 12 insertions(+), 27 deletions(-)

diff --git a/mm/page_table_check.c b/mm/page_table_check.c
index 509c6ef8de400..e11bebf23e36f 100644
--- a/mm/page_table_check.c
+++ b/mm/page_table_check.c
@@ -62,24 +62,20 @@ static struct page_table_check *get_page_table_check(struct page_ext *page_ext)
  */
 static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
 {
+	struct page_ext_iter iter;
 	struct page_ext *page_ext;
 	struct page *page;
-	unsigned long i;
 	bool anon;
 
 	if (!pfn_valid(pfn))
 		return;
 
 	page = pfn_to_page(pfn);
-	page_ext = page_ext_get(page);
-
-	if (!page_ext)
-		return;
-
 	BUG_ON(PageSlab(page));
 	anon = PageAnon(page);
 
-	for (i = 0; i < pgcnt; i++) {
+	rcu_read_lock();
+	for_each_page_ext(page, pgcnt, page_ext, iter) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
 		if (anon) {
@@ -89,9 +85,8 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
 			BUG_ON(atomic_read(&ptc->anon_map_count));
 			BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
 		}
-		page_ext = page_ext_next(page_ext);
 	}
-	page_ext_put(page_ext);
+	rcu_read_unlock();
 }
 
 /*
@@ -102,24 +97,20 @@ static void page_table_check_clear(unsigned long pfn, unsigned long pgcnt)
 static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
 				 bool rw)
 {
+	struct page_ext_iter iter;
 	struct page_ext *page_ext;
 	struct page *page;
-	unsigned long i;
 	bool anon;
 
 	if (!pfn_valid(pfn))
 		return;
 
 	page = pfn_to_page(pfn);
-	page_ext = page_ext_get(page);
-
-	if (!page_ext)
-		return;
-
 	BUG_ON(PageSlab(page));
 	anon = PageAnon(page);
 
-	for (i = 0; i < pgcnt; i++) {
+	rcu_read_lock();
+	for_each_page_ext(page, pgcnt, page_ext, iter) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
 		if (anon) {
@@ -129,9 +120,8 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
 			BUG_ON(atomic_read(&ptc->anon_map_count));
 			BUG_ON(atomic_inc_return(&ptc->file_map_count) < 0);
 		}
-		page_ext = page_ext_next(page_ext);
 	}
-	page_ext_put(page_ext);
+	rcu_read_unlock();
 }
 
 /*
@@ -140,24 +130,19 @@ static void page_table_check_set(unsigned long pfn, unsigned long pgcnt,
  */
 void __page_table_check_zero(struct page *page, unsigned int order)
 {
+	struct page_ext_iter iter;
 	struct page_ext *page_ext;
-	unsigned long i;
 
 	BUG_ON(PageSlab(page));
 
-	page_ext = page_ext_get(page);
-
-	if (!page_ext)
-		return;
-
-	for (i = 0; i < (1ul << order); i++) {
+	rcu_read_lock();
+	for_each_page_ext(page, 1 << order, page_ext, iter) {
 		struct page_table_check *ptc = get_page_table_check(page_ext);
 
 		BUG_ON(atomic_read(&ptc->anon_map_count));
 		BUG_ON(atomic_read(&ptc->file_map_count));
-		page_ext = page_ext_next(page_ext);
 	}
-	page_ext_put(page_ext);
+	rcu_read_unlock();
 }
 
 void __page_table_check_pte_clear(struct mm_struct *mm, pte_t pte)
-- 
2.48.1

[PATCH v3 3/3] mm: page_owner: use new iteration API

[Luiz Capitulino]

The page_ext_next() function assumes that page extension objects for a
page order allocation always reside in the same memory section, which
may not be true and could lead to crashes. Use the new page_ext
iteration API instead.

Fixes: cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
---
 mm/page_owner.c | 84 +++++++++++++++++++++++--------------------------
 1 file changed, 39 insertions(+), 45 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index 2d6360eaccbb6..65adc66582d82 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -229,17 +229,19 @@ static void dec_stack_record_count(depot_stack_handle_t handle,
 			handle);
 }
 
-static inline void __update_page_owner_handle(struct page_ext *page_ext,
+static inline void __update_page_owner_handle(struct page *page,
 					      depot_stack_handle_t handle,
 					      unsigned short order,
 					      gfp_t gfp_mask,
 					      short last_migrate_reason, u64 ts_nsec,
 					      pid_t pid, pid_t tgid, char *comm)
 {
-	int i;
+	struct page_ext_iter iter;
+	struct page_ext *page_ext;
 	struct page_owner *page_owner;
 
-	for (i = 0; i < (1 << order); i++) {
+	rcu_read_lock();
+	for_each_page_ext(page, 1 << order, page_ext, iter) {
 		page_owner = get_page_owner(page_ext);
 		page_owner->handle = handle;
 		page_owner->order = order;
@@ -252,20 +254,22 @@ static inline void __update_page_owner_handle(struct page_ext *page_ext,
 			sizeof(page_owner->comm));
 		__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
 		__set_bit(PAGE_EXT_OWNER_ALLOCATED, &page_ext->flags);
-		page_ext = page_ext_next(page_ext);
 	}
+	rcu_read_unlock();
 }
 
-static inline void __update_page_owner_free_handle(struct page_ext *page_ext,
+static inline void __update_page_owner_free_handle(struct page *page,
 						   depot_stack_handle_t handle,
 						   unsigned short order,
 						   pid_t pid, pid_t tgid,
 						   u64 free_ts_nsec)
 {
-	int i;
+	struct page_ext_iter iter;
+	struct page_ext *page_ext;
 	struct page_owner *page_owner;
 
-	for (i = 0; i < (1 << order); i++) {
+	rcu_read_lock();
+	for_each_page_ext(page, 1 << order, page_ext, iter) {
 		page_owner = get_page_owner(page_ext);
 		/* Only __reset_page_owner() wants to clear the bit */
 		if (handle) {
@@ -275,8 +279,8 @@ static inline void __update_page_owner_free_handle(struct page_ext *page_ext,
 		page_owner->free_ts_nsec = free_ts_nsec;
 		page_owner->free_pid = current->pid;
 		page_owner->free_tgid = current->tgid;
-		page_ext = page_ext_next(page_ext);
 	}
+	rcu_read_unlock();
 }
 
 void __reset_page_owner(struct page *page, unsigned short order)
@@ -293,11 +297,11 @@ void __reset_page_owner(struct page *page, unsigned short order)
 
 	page_owner = get_page_owner(page_ext);
 	alloc_handle = page_owner->handle;
+	page_ext_put(page_ext);
 
 	handle = save_stack(GFP_NOWAIT | __GFP_NOWARN);
-	__update_page_owner_free_handle(page_ext, handle, order, current->pid,
+	__update_page_owner_free_handle(page, handle, order, current->pid,
 					current->tgid, free_ts_nsec);
-	page_ext_put(page_ext);
 
 	if (alloc_handle != early_handle)
 		/*
@@ -313,19 +317,13 @@ void __reset_page_owner(struct page *page, unsigned short order)
 noinline void __set_page_owner(struct page *page, unsigned short order,
 					gfp_t gfp_mask)
 {
-	struct page_ext *page_ext;
 	u64 ts_nsec = local_clock();
 	depot_stack_handle_t handle;
 
 	handle = save_stack(gfp_mask);
-
-	page_ext = page_ext_get(page);
-	if (unlikely(!page_ext))
-		return;
-	__update_page_owner_handle(page_ext, handle, order, gfp_mask, -1,
+	__update_page_owner_handle(page, handle, order, gfp_mask, -1,
 				   ts_nsec, current->pid, current->tgid,
 				   current->comm);
-	page_ext_put(page_ext);
 	inc_stack_record_count(handle, gfp_mask, 1 << order);
 }
 
@@ -344,44 +342,42 @@ void __set_page_owner_migrate_reason(struct page *page, int reason)
 
 void __split_page_owner(struct page *page, int old_order, int new_order)
 {
-	int i;
-	struct page_ext *page_ext = page_ext_get(page);
+	struct page_ext_iter iter;
+	struct page_ext *page_ext;
 	struct page_owner *page_owner;
 
-	if (unlikely(!page_ext))
-		return;
-
-	for (i = 0; i < (1 << old_order); i++) {
+	rcu_read_lock();
+	for_each_page_ext(page, 1 << old_order, page_ext, iter) {
 		page_owner = get_page_owner(page_ext);
 		page_owner->order = new_order;
-		page_ext = page_ext_next(page_ext);
 	}
-	page_ext_put(page_ext);
+	rcu_read_unlock();
 }
 
 void __folio_copy_owner(struct folio *newfolio, struct folio *old)
 {
-	int i;
-	struct page_ext *old_ext;
-	struct page_ext *new_ext;
+	struct page_ext *page_ext;
+	struct page_ext_iter iter;
 	struct page_owner *old_page_owner;
 	struct page_owner *new_page_owner;
 	depot_stack_handle_t migrate_handle;
 
-	old_ext = page_ext_get(&old->page);
-	if (unlikely(!old_ext))
+	page_ext = page_ext_get(&old->page);
+	if (unlikely(!page_ext))
 		return;
 
-	new_ext = page_ext_get(&newfolio->page);
-	if (unlikely(!new_ext)) {
-		page_ext_put(old_ext);
+	old_page_owner = get_page_owner(page_ext);
+	page_ext_put(page_ext);
+
+	page_ext = page_ext_get(&newfolio->page);
+	if (unlikely(!page_ext))
 		return;
-	}
 
-	old_page_owner = get_page_owner(old_ext);
-	new_page_owner = get_page_owner(new_ext);
+	new_page_owner = get_page_owner(page_ext);
+	page_ext_put(page_ext);
+
 	migrate_handle = new_page_owner->handle;
-	__update_page_owner_handle(new_ext, old_page_owner->handle,
+	__update_page_owner_handle(&newfolio->page, old_page_owner->handle,
 				   old_page_owner->order, old_page_owner->gfp_mask,
 				   old_page_owner->last_migrate_reason,
 				   old_page_owner->ts_nsec, old_page_owner->pid,
@@ -391,7 +387,7 @@ void __folio_copy_owner(struct folio *newfolio, struct folio *old)
 	 * will be freed after migration. Keep them until then as they may be
 	 * useful.
 	 */
-	__update_page_owner_free_handle(new_ext, 0, old_page_owner->order,
+	__update_page_owner_free_handle(&newfolio->page, 0, old_page_owner->order,
 					old_page_owner->free_pid,
 					old_page_owner->free_tgid,
 					old_page_owner->free_ts_nsec);
@@ -400,14 +396,12 @@ void __folio_copy_owner(struct folio *newfolio, struct folio *old)
 	 * for the new one and the old folio otherwise there will be an imbalance
 	 * when subtracting those pages from the stack.
 	 */
-	for (i = 0; i < (1 << new_page_owner->order); i++) {
+	rcu_read_lock();
+	for_each_page_ext(&old->page, 1 << new_page_owner->order, page_ext, iter) {
+		old_page_owner = get_page_owner(page_ext);
 		old_page_owner->handle = migrate_handle;
-		old_ext = page_ext_next(old_ext);
-		old_page_owner = get_page_owner(old_ext);
 	}
-
-	page_ext_put(new_ext);
-	page_ext_put(old_ext);
+	rcu_read_unlock();
 }
 
 void pagetypeinfo_showmixedcount_print(struct seq_file *m,
@@ -813,7 +807,7 @@ static void init_pages_in_zone(pg_data_t *pgdat, struct zone *zone)
 				goto ext_put_continue;
 
 			/* Found early allocated page */
-			__update_page_owner_handle(page_ext, early_handle, 0, 0,
+			__update_page_owner_handle(page, early_handle, 0, 0,
 						   -1, local_clock(), current->pid,
 						   current->tgid, current->comm);
 			count++;
-- 
2.48.1

Re: [PATCH v3 3/3] mm: page_owner: use new iteration API

[David Hildenbrand]

On 06.03.25 23:44, Luiz Capitulino wrote:
> The page_ext_next() function assumes that page extension objects for a
> page order allocation always reside in the same memory section, which
> may not be true and could lead to crashes. Use the new page_ext
> iteration API instead.
> 
> Fixes: cf54f310d0d3 ("mm/hugetlb: use __GFP_COMP for gigantic folios")
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>   mm/page_owner.c | 84 +++++++++++++++++++++++--------------------------
>   1 file changed, 39 insertions(+), 45 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index 2d6360eaccbb6..65adc66582d82 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -229,17 +229,19 @@ static void dec_stack_record_count(depot_stack_handle_t handle,
>   			handle);
>   }
>   
> -static inline void __update_page_owner_handle(struct page_ext *page_ext,
> +static inline void __update_page_owner_handle(struct page *page,
>   					      depot_stack_handle_t handle,
>   					      unsigned short order,
>   					      gfp_t gfp_mask,
>   					      short last_migrate_reason, u64 ts_nsec,
>   					      pid_t pid, pid_t tgid, char *comm)
>   {
> -	int i;
> +	struct page_ext_iter iter;
> +	struct page_ext *page_ext;
>   	struct page_owner *page_owner;
>   
> -	for (i = 0; i < (1 << order); i++) {
> +	rcu_read_lock();
> +	for_each_page_ext(page, 1 << order, page_ext, iter) {

I realize that we could get rid of all the temporary
page_ext variables and simply operate on iter.page_ext.

If that makes sense, it could be done as a cleanup on top later. Thanks 
Luiz!

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb