[PATCH v3] mm/page_owner.c: add llseek for page_owner

[Kassey Li <quic_yingangl@quicinc.com>]

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 (SUSE) <vbabka@kernel.org>
Signed-off-by: Kassey Li <quic_yingangl@quicinc.com>
---
 mm/page_owner.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index e4c6f3f1695b..231b1877af99 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -497,8 +497,8 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 		return -EINVAL;
 
 	page = NULL;
-	pfn = min_low_pfn + *ppos;
 
+	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 +561,7 @@ 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);
@@ -570,6 +570,21 @@ read_page_owner(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 	return 0;
 }
 
+static loff_t llseek_page_owner(struct file *file, loff_t offset, int whence)
+{
+	loff_t retval = 0;
+	switch (whence) {
+		case SEEK_CUR:
+		case SEEK_SET:
+			file->f_pos = offset;
+			break;
+		default:
+			retval = -ENXIO;
+	}
+
+	return retval;
+}
+
 static void init_pages_in_zone(pg_data_t *pgdat, struct zone *zone)
 {
 	unsigned long pfn = zone->zone_start_pfn;
@@ -660,6 +675,7 @@ static void init_early_allocated_pages(void)
 
 static const struct file_operations proc_page_owner_operations = {
 	.read		= read_page_owner,
+	.llseek 	= llseek_page_owner,
 };
 
 static int __init pageowner_init(void)
-- 
2.17.1