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 7e2f2a0 ("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/[email protected]
Fixes: 7e2f2a0 ("mm, page_owner: record page owner for each subpage")
Signed-off-by: Vlastimil Babka <[email protected]>
Reported-by: Kirill A. Shutemov <[email protected]>
Reported-by: Miles Chen <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Walter Wu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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;
 

mm/page_ext.c

@@ -67,8 +67,9 @@ static struct page_ext_operations *page_ext_ops[] = {
 #endif
 };
 
+unsigned long page_ext_size = sizeof(struct page_ext);
+
 static unsigned long total_usage;
-static unsigned long extra_mem;
 
 static bool __init invoke_need_callbacks(void)
 {
@@ -78,9 +79,8 @@ static bool __init invoke_need_callbacks(void)
 
 	for (i = 0; i < entries; i++) {
 		if (page_ext_ops[i]->need && page_ext_ops[i]->need()) {
-			page_ext_ops[i]->offset = sizeof(struct page_ext) +
-						extra_mem;
-			extra_mem += page_ext_ops[i]->size;
+			page_ext_ops[i]->offset = page_ext_size;
+			page_ext_size += page_ext_ops[i]->size;
 			need = true;
 		}
 	}
@@ -99,14 +99,9 @@ static void __init invoke_init_callbacks(void)
 	}
 }
 
-static unsigned long get_entry_size(void)
-{
-	return sizeof(struct page_ext) + extra_mem;
-}
-
 static inline struct page_ext *get_entry(void *base, unsigned long index)
 {
-	return base + get_entry_size() * index;
+	return base + page_ext_size * index;
 }
 
 #if !defined(CONFIG_SPARSEMEM)
@@ -156,7 +151,7 @@ static int __init alloc_node_page_ext(int nid)
 		!IS_ALIGNED(node_end_pfn(nid), MAX_ORDER_NR_PAGES))
 		nr_pages += MAX_ORDER_NR_PAGES;
 
-	table_size = get_entry_size() * nr_pages;
+	table_size = page_ext_size * nr_pages;
 
 	base = memblock_alloc_try_nid(
 			table_size, PAGE_SIZE, __pa(MAX_DMA_ADDRESS),
@@ -234,7 +229,7 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
 	if (section->page_ext)
 		return 0;
 
-	table_size = get_entry_size() * PAGES_PER_SECTION;
+	table_size = page_ext_size * PAGES_PER_SECTION;
 	base = alloc_page_ext(table_size, nid);
 
 	/*
@@ -254,7 +249,7 @@ static int __meminit init_section_page_ext(unsigned long pfn, int nid)
 	 * we need to apply a mask.
 	 */
 	pfn &= PAGE_SECTION_MASK;
-	section->page_ext = (void *)base - get_entry_size() * pfn;
+	section->page_ext = (void *)base - page_ext_size * pfn;
 	total_usage += table_size;
 	return 0;
 }
@@ -267,7 +262,7 @@ static void free_page_ext(void *addr)
 		struct page *page = virt_to_page(addr);
 		size_t table_size;
 
-		table_size = get_entry_size() * PAGES_PER_SECTION;
+		table_size = page_ext_size * PAGES_PER_SECTION;
 
 		BUG_ON(PageReserved(page));
 		kmemleak_free(addr);

mm/page_owner.c

@@ -156,17 +156,18 @@ 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()) {
 			page_owner = get_page_owner(page_ext);
 			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);
 	}
 }