mm, page_owner: fix off-by-one error in __set_page_owner_handle()

Patch series "followups to debug_pagealloc improvements through
page_owner", v3.

These are followups to [1] which made it to Linus meanwhile.  Patches 1
and 3 are based on Kirill's review, patch 2 on KASAN request [2].  It
would be nice if all of this made it to 5.4 with [1] already there (or
at least Patch 1).

This patch (of 3):

As noted by Kirill, commit 7e2f2a0cd17c ("mm, page_owner: record page
owner for each subpage") has introduced an off-by-one error in
__set_page_owner_handle() when looking up page_ext for subpages.  As a
result, the head page page_owner info is set twice, while for the last
tail page, it's not set at all.

Fix this and also make the code more efficient by advancing the page_ext
pointer we already have, instead of calling lookup_page_ext() for each
subpage.  Since the full size of struct page_ext is not known at compile
time, we can't use a simple page_ext++ statement, so introduce a
page_ext_next() inline function for that.

Link: http://lkml.kernel.org/r/20190930122916.14969-2-vbabka@suse.cz
Fixes: 7e2f2a0cd17c ("mm, page_owner: record page owner for each subpage")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Kirill A. Shutemov <kirill@shutemov.name>
Reported-by: Miles Chen <miles.chen@mediatek.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Walter Wu <walter-zh.wu@mediatek.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/linux/page_ext.h b/include/linux/page_ext.h
index 682fd465df06..5e856512bafb 100644
--- a/include/linux/page_ext.h
+++ b/include/linux/page_ext.h
@@ -36,6 +36,7 @@ struct page_ext {
 	unsigned long flags;
 };
 
+extern unsigned long page_ext_size;
 extern void pgdat_page_ext_init(struct pglist_data *pgdat);
 
 #ifdef CONFIG_SPARSEMEM
@@ -52,6 +53,13 @@ static inline void page_ext_init(void)
 
 struct page_ext *lookup_page_ext(const struct page *page);
 
+static inline struct page_ext *page_ext_next(struct page_ext *curr)
+{
+	void *next = curr;
+	next += page_ext_size;
+	return next;
+}
+
 #else /* !CONFIG_PAGE_EXTENSION */
 struct page_ext;