[PATCH v6] mm/page_owner.c: add llseek for page_owner

[Kassey Li]

There is usage to dump a given cma region page_owner
instead of all page's.

This change allows to specify a ppos as start_pfn
by fseek.

Any invalid ppos will be skipped, so it did not
broken the origin dump feature.

Suggested-by: Vlastimil Babka <vbabka@xxxxxxx>
Signed-off-by: Kassey Li <quic_yingangl@xxxxxxxxxxx>
---
 Documentation/vm/page_owner.rst |  6 ++++++
 mm/page_owner.c                 | 10 +++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/Documentation/vm/page_owner.rst b/Documentation/vm/page_owner.rst
index f5c954afe97c..8d33d976da8a 100644
--- a/Documentation/vm/page_owner.rst
+++ b/Documentation/vm/page_owner.rst
@@ -95,6 +95,12 @@ Usage
 	PFN XXX ...
 	// Detailed stack
 
+   By default, it will do full pfn dump, to start with a given pfn,
+   page_owner supports fseek.
+
+   FILE *fp = fopen("/sys/kernel/debug/page_owner", "r");
+   fseek(fp, pfn_start, SEEK_SET);
+
    The ``page_owner_sort`` tool ignores ``PFN`` rows, puts the remaining rows
    in buf, uses regexp to extract the page order value, counts the times
    and pages of buf, and finally sorts them according to the parameter(s).
diff --git a/mm/page_owner.c b/mm/page_owner.c
index e4c6f3f1695b..33832b5908e6 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -13,6 +13,7 @@
 #include <linux/memcontrol.h>
 #include <linux/sched/clock.h>
 
+#include "../fs/proc/internal.h"
 #include "internal.h"
 
 /*
@@ -497,8 +498,10 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		return -EINVAL;
 
 	page = NULL;
-	pfn = min_low_pfn + *ppos;
-
+	if (*ppos == 0)
+		pfn = min_low_pfn;
+	else
+		pfn = *ppos;
 	/* Find a valid PFN or the start of a MAX_ORDER_NR_PAGES area */
 	while (!pfn_valid(pfn) && (pfn & (MAX_ORDER_NR_PAGES - 1)) != 0)
 		pfn++;
@@ -561,7 +564,7 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 			continue;
 
 		/* Record the next PFN to read in the file offset */
-		*ppos = (pfn - min_low_pfn) + 1;
+		*ppos = pfn + 1;
 
 		return print_page_owner(buf, count, pfn, page,
 				page_owner, handle);
@@ -660,6 +663,7 @@ static void init_early_allocated_pages(void)
 
 static const struct file_operations proc_page_owner_operations = {
 	.read		= read_page_owner,
+	.llseek		= mem_lseek,
 };
 
 static int __init pageowner_init(void)
-- 
2.17.1