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 7e2f2a0cd1 ("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: 7e2f2a0cd1 ("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>

mm/page_owner.c

@@ -156,10 +156,10 @@ void __reset_page_owner(struct page *page, unsigned int order)
 		handle = save_stack(GFP_NOWAIT | __GFP_NOWARN);
 #endif
 
+	page_ext = lookup_page_ext(page);
+	if (unlikely(!page_ext))
+		return;
 	for (i = 0; i < (1 << order); i++) {
-		page_ext = lookup_page_ext(page + i);
-		if (unlikely(!page_ext))
-			continue;
 		__clear_bit(PAGE_EXT_OWNER_ACTIVE, &page_ext->flags);
 #ifdef CONFIG_DEBUG_PAGEALLOC
 		if (debug_pagealloc_enabled()) {
@@ -167,6 +167,7 @@ void __reset_page_owner(struct page *page, unsigned int order)
 			page_owner->free_handle = handle;
 		}
 #endif
+		page_ext = page_ext_next(page_ext);
 	}
 }
 
@@ -186,7 +187,7 @@ static inline void __set_page_owner_handle(struct page *page,
 		__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
 		__set_bit(PAGE_EXT_OWNER_ACTIVE, &page_ext->flags);
 
-		page_ext = lookup_page_ext(page + i);
+		page_ext = page_ext_next(page_ext);
 	}
 }
 
@@ -224,12 +225,10 @@ void __split_page_owner(struct page *page, unsigned int order)
 	if (unlikely(!page_ext))
 		return;
 
-	page_owner = get_page_owner(page_ext);
-	page_owner->order = 0;
-	for (i = 1; i < (1 << order); i++) {
-		page_ext = lookup_page_ext(page + i);
+	for (i = 0; i < (1 << order); i++) {
 		page_owner = get_page_owner(page_ext);
 		page_owner->order = 0;
+		page_ext = page_ext_next(page_ext);
 	}
 }