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Subject[PATCH 6/6] mm/page_owner: use stackdepot to store stacktrace
Date
From: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Currently, we store each page's allocation stacktrace on corresponding
page_ext structure and it requires a lot of memory. This causes the problem
that memory tight system doesn't work well if page_owner is enabled.
Moreover, even with this large memory consumption, we cannot get full
stacktrace because we allocate memory at boot time and just maintain
8 stacktrace slots to balance memory consumption. We could increase it
to more but it would make system unusable or change system behaviour.

To solve the problem, this patch uses stackdepot to store stacktrace.
It obviously provides memory saving but there is a drawback that
stackdepot could fail.

stackdepot allocates memory at runtime so it could fail if system has
not enough memory. But, most of allocation stack are generated at very
early time and there are much memory at this time. So, failure would not
happen easily. And, one failure means that we miss just one page's
allocation stacktrace so it would not be a big problem. In this patch,
when memory allocation failure happens, we store special stracktrace
handle to the page that is failed to save stacktrace. With it, user
can guess memory usage properly even if failure happens.

Memory saving looks as following. (Boot 4GB memory system with page_owner)

92274688 bytes -> 25165824 bytes

72% reduction in static allocation size. Even if we should add up size of
dynamic allocation memory, it would not that big because stacktrace is
mostly duplicated.

Note that implementation looks complex than someone would imagine because
there is recursion issue. stackdepot uses page allocator and page_owner
is called at page allocation. Using stackdepot in page_owner could re-call
page allcator and then page_owner. That is a recursion. To detect and
avoid it, whenever we obtain stacktrace, recursion is checked and
page_owner is set to dummy information if found. Dummy information means
that this page is allocated for page_owner feature itself
(such as stackdepot) and it's understandable behavior for user.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
---
include/linux/page_ext.h | 4 +-
lib/Kconfig.debug | 1 +
mm/page_owner.c | 128 ++++++++++++++++++++++++++++++++++++++++-------
3 files changed, 114 insertions(+), 19 deletions(-)

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index e1fe7cf..03f2a3e 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -3,6 +3,7 @@

#include <linux/types.h>
#include <linux/stacktrace.h>
+#include <linux/stackdepot.h>

struct pglist_data;
struct page_ext_operations {
@@ -44,9 +45,8 @@ struct page_ext {
#ifdef CONFIG_PAGE_OWNER
unsigned int order;
gfp_t gfp_mask;
- unsigned int nr_entries;
int last_migrate_reason;
- unsigned long trace_entries[8];
+ depot_stack_handle_t handle;
#endif
};

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 5d57177..a32fd24 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -248,6 +248,7 @@ config PAGE_OWNER
depends on DEBUG_KERNEL && STACKTRACE_SUPPORT
select DEBUG_FS
select STACKTRACE
+ select STACKDEPOT
select PAGE_EXTENSION
help
This keeps track of what call chain is the owner of a page, may
diff --git a/mm/page_owner.c b/mm/page_owner.c
index 7b5a834..7875de5 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -7,11 +7,18 @@
#include <linux/page_owner.h>
#include <linux/jump_label.h>
#include <linux/migrate.h>
+#include <linux/stackdepot.h>
+
#include "internal.h"

+#define PAGE_OWNER_STACK_DEPTH (64)
+
static bool page_owner_disabled = true;
DEFINE_STATIC_KEY_FALSE(page_owner_inited);

+static depot_stack_handle_t dummy_handle;
+static depot_stack_handle_t failure_handle;
+
static void init_early_allocated_pages(void);

static int early_page_owner_param(char *buf)
@@ -34,11 +41,41 @@ static bool need_page_owner(void)
return true;
}

+static noinline void register_dummy_stack(void)
+{
+ unsigned long entries[4];
+ struct stack_trace dummy;
+
+ dummy.nr_entries = 0;
+ dummy.max_entries = ARRAY_SIZE(entries);
+ dummy.entries = &entries[0];
+ dummy.skip = 0;
+
+ save_stack_trace(&dummy);
+ dummy_handle = depot_save_stack(&dummy, GFP_KERNEL);
+}
+
+static noinline void register_failure_stack(void)
+{
+ unsigned long entries[4];
+ struct stack_trace failure;
+
+ failure.nr_entries = 0;
+ failure.max_entries = ARRAY_SIZE(entries);
+ failure.entries = &entries[0];
+ failure.skip = 0;
+
+ save_stack_trace(&failure);
+ failure_handle = depot_save_stack(&failure, GFP_KERNEL);
+}
+
static void init_page_owner(void)
{
if (page_owner_disabled)
return;

+ register_dummy_stack();
+ register_failure_stack();
static_branch_enable(&page_owner_inited);
init_early_allocated_pages();
}
@@ -59,21 +96,56 @@ void __reset_page_owner(struct page *page, unsigned int order)
}
}

-void __set_page_owner(struct page *page, unsigned int order, gfp_t gfp_mask)
+static inline bool check_recursive_alloc(struct stack_trace *trace,
+ unsigned long ip)
{
- struct page_ext *page_ext = lookup_page_ext(page);
+ int i, count;
+
+ if (!trace->nr_entries)
+ return false;
+
+ for (i = 0, count = 0; i < trace->nr_entries; i++) {
+ if (trace->entries[i] == ip && ++count == 2)
+ return true;
+ }
+
+ return false;
+}
+
+static noinline depot_stack_handle_t save_stack(gfp_t flags)
+{
+ unsigned long entries[PAGE_OWNER_STACK_DEPTH];
struct stack_trace trace = {
.nr_entries = 0,
- .max_entries = ARRAY_SIZE(page_ext->trace_entries),
- .entries = &page_ext->trace_entries[0],
- .skip = 3,
+ .entries = entries,
+ .max_entries = PAGE_OWNER_STACK_DEPTH,
+ .skip = 0
};
+ depot_stack_handle_t handle;

save_stack_trace(&trace);
+ if (trace.nr_entries != 0 &&
+ trace.entries[trace.nr_entries-1] == ULONG_MAX)
+ trace.nr_entries--;
+
+ if (check_recursive_alloc(&trace, _RET_IP_))
+ return dummy_handle;
+
+ handle = depot_save_stack(&trace, flags);
+ if (!handle)
+ handle = failure_handle;
+
+ return handle;
+}

+noinline void __set_page_owner(struct page *page, unsigned int order,
+ gfp_t gfp_mask)
+{
+ struct page_ext *page_ext = lookup_page_ext(page);
+
+ page_ext->handle = save_stack(gfp_mask);
page_ext->order = order;
page_ext->gfp_mask = gfp_mask;
- page_ext->nr_entries = trace.nr_entries;
page_ext->last_migrate_reason = -1;

__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
@@ -100,15 +172,11 @@ void __copy_page_owner(struct page *oldpage, struct page *newpage)
{
struct page_ext *old_ext = lookup_page_ext(oldpage);
struct page_ext *new_ext = lookup_page_ext(newpage);
- int i;

new_ext->order = old_ext->order;
new_ext->gfp_mask = old_ext->gfp_mask;
new_ext->last_migrate_reason = old_ext->last_migrate_reason;
- new_ext->nr_entries = old_ext->nr_entries;
-
- for (i = 0; i < ARRAY_SIZE(new_ext->trace_entries); i++)
- new_ext->trace_entries[i] = old_ext->trace_entries[i];
+ new_ext->handle = old_ext->handle;

/*
* We don't clear the bit on the oldpage as it's going to be freed
@@ -124,14 +192,18 @@ void __copy_page_owner(struct page *oldpage, struct page *newpage)

static ssize_t
print_page_owner(char __user *buf, size_t count, unsigned long pfn,
- struct page *page, struct page_ext *page_ext)
+ struct page *page, struct page_ext *page_ext,
+ depot_stack_handle_t handle)
{
int ret;
int pageblock_mt, page_mt;
char *kbuf;
+ unsigned long entries[PAGE_OWNER_STACK_DEPTH];
struct stack_trace trace = {
- .nr_entries = page_ext->nr_entries,
- .entries = &page_ext->trace_entries[0],
+ .nr_entries = 0,
+ .entries = entries,
+ .max_entries = PAGE_OWNER_STACK_DEPTH,
+ .skip = 0
};

kbuf = kmalloc(count, GFP_KERNEL);
@@ -160,6 +232,7 @@ print_page_owner(char __user *buf, size_t count, unsigned long pfn,
if (ret >= count)
goto err;

+ depot_fetch_stack(handle, &trace);
ret += snprint_stack_trace(kbuf + ret, count - ret, &trace, 0);
if (ret >= count)
goto err;
@@ -190,10 +263,14 @@ err:
void __dump_page_owner(struct page *page)
{
struct page_ext *page_ext = lookup_page_ext(page);
+ unsigned long entries[PAGE_OWNER_STACK_DEPTH];
struct stack_trace trace = {
- .nr_entries = page_ext->nr_entries,
- .entries = &page_ext->trace_entries[0],
+ .nr_entries = 0,
+ .entries = entries,
+ .max_entries = PAGE_OWNER_STACK_DEPTH,
+ .skip = 0
};
+ depot_stack_handle_t handle;
gfp_t gfp_mask = page_ext->gfp_mask;
int mt = gfpflags_to_migratetype(gfp_mask);

@@ -202,6 +279,13 @@ void __dump_page_owner(struct page *page)
return;
}

+ handle = READ_ONCE(page_ext->handle);
+ if (!handle) {
+ pr_alert("page_owner info is not active (free page?)\n");
+ return;
+ }
+
+ depot_fetch_stack(handle, &trace);
pr_alert("page allocated via order %u, migratetype %s, gfp_mask %#x(%pGg)\n",
page_ext->order, migratetype_names[mt], gfp_mask, &gfp_mask);
print_stack_trace(&trace, 0);
@@ -217,6 +301,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
unsigned long pfn;
struct page *page;
struct page_ext *page_ext;
+ depot_stack_handle_t handle;

if (!static_branch_unlikely(&page_owner_inited))
return -EINVAL;
@@ -263,10 +348,19 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
if (!test_bit(PAGE_EXT_OWNER, &page_ext->flags))
continue;

+ /*
+ * Access to page_ext->handle isn't synchronous so we should
+ * be careful to access it.
+ */
+ handle = READ_ONCE(page_ext->handle);
+ if (!handle)
+ continue;
+
/* Record the next PFN to read in the file offset */
*ppos = (pfn - min_low_pfn) + 1;

- return print_page_owner(buf, count, pfn, page, page_ext);
+ return print_page_owner(buf, count, pfn, page,
+ page_ext, handle);
}

return 0;
--
1.9.1
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