[PATCH 1/2] mm: Replace zero-length array with flexible-array member

[PATCH 1/2] mm: Replace zero-length array with flexible-array member

[qiwuchen55]

From: chenqiwu <chenqiwu@xiaomi.com>

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
 include/linux/mm.h     | 2 +-
 include/linux/mmzone.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index c54fb96..8dc699a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1528,7 +1528,7 @@ struct frame_vector {
 	unsigned int nr_frames;	/* Number of frames stored in ptrs array */
 	bool got_ref;		/* Did we pin pages by getting page ref? */
 	bool is_pfns;		/* Does array contain pages or pfns? */
-	void *ptrs[0];		/* Array of pinned pfns / pages. Use
+	void *ptrs[];		/* Array of pinned pfns / pages. Use
 				 * pfns_vector_pages() or pfns_vector_pfns()
 				 * for access */
 };
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 462f687..0e3c685 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1187,7 +1187,7 @@ static inline unsigned long section_nr_to_pfn(unsigned long sec)
 struct mem_section_usage {
 	DECLARE_BITMAP(subsection_map, SUBSECTIONS_PER_SECTION);
 	/* See declaration of similar field in struct zone */
-	unsigned long pageblock_flags[0];
+	unsigned long pageblock_flags[];
 };
 
 void subsection_map_init(unsigned long pfn, unsigned long nr_pages);
-- 
1.9.1

[PATCH 2/2] mm/vmscan: make several optimizations for isolate_lru_pages()

[qiwuchen55]

From: chenqiwu <chenqiwu@xiaomi.com>

1) Simplify the code for initializing some variables.
1) Add const modifier to decorate the mode variable.
2) Define a page_zoneidx variable to reduce the redundant code
of page_zonenum().

Signed-off-by: chenqiwu <chenqiwu@xiaomi.com>
---
 mm/vmscan.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8763705..cf90e8b 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1639,16 +1639,15 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 	struct list_head *src = &lruvec->lists[lru];
 	unsigned long nr_taken = 0;
 	unsigned long nr_zone_taken[MAX_NR_ZONES] = { 0 };
-	unsigned long nr_skipped[MAX_NR_ZONES] = { 0, };
+	unsigned long nr_skipped[MAX_NR_ZONES] = { 0 };
 	unsigned long skipped = 0;
-	unsigned long scan, total_scan, nr_pages;
+	unsigned long scan = 0, total_scan = 0, nr_pages;
 	LIST_HEAD(pages_skipped);
-	isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
+	const isolate_mode_t mode = (sc->may_unmap ? 0 : ISOLATE_UNMAPPED);
 
-	total_scan = 0;
-	scan = 0;
 	while (scan < nr_to_scan && !list_empty(src)) {
 		struct page *page;
+		enum zone_type page_zoneidx;
 
 		page = lru_to_page(src);
 		prefetchw_prev_lru_page(page, src, flags);
@@ -1658,9 +1657,10 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 		nr_pages = compound_nr(page);
 		total_scan += nr_pages;
 
-		if (page_zonenum(page) > sc->reclaim_idx) {
+		page_zoneidx = page_zonenum(page);
+		if (page_zoneidx > sc->reclaim_idx) {
 			list_move(&page->lru, &pages_skipped);
-			nr_skipped[page_zonenum(page)] += nr_pages;
+			nr_skipped[page_zoneidx] += nr_pages;
 			continue;
 		}
 
@@ -1678,7 +1678,7 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 		switch (__isolate_lru_page(page, mode)) {
 		case 0:
 			nr_taken += nr_pages;
-			nr_zone_taken[page_zonenum(page)] += nr_pages;
+			nr_zone_taken[page_zoneidx] += nr_pages;
 			list_move(&page->lru, dst);
 			break;
 
-- 
1.9.1