[PATCH RFC 06/10] mm: page_frag: reuse MSB of 'size' field for pfmemalloc

[Yunsheng Lin <linyunsheng@xxxxxxxxxx>]

The '(PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)' case is for the
system with page size less than 32KB, which is 0x8000 bytes
requiring 16 bits space, change 'size' to 'size_mask' to avoid
using the MSB, and change 'pfmemalloc' field to reuse the that
MSB, so that we remove the orginal space needed by 'pfmemalloc'.

For another case, the MSB of 'offset' is reused for 'pfmemalloc'.

Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>
---
 include/linux/page_frag_cache.h | 13 ++++++++-----
 mm/page_frag_alloc.c            |  5 +++--
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h
index fe5faa80b6c3..40a7d6da9ef0 100644
--- a/include/linux/page_frag_cache.h
+++ b/include/linux/page_frag_cache.h
@@ -12,15 +12,16 @@ struct page_frag_cache {
 	void *va;
 #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
 	__u16 offset;
-	__u16 size;
+	__u16 size_mask:15;
+	__u16 pfmemalloc:1;
 #else
-	__u32 offset;
+	__u32 offset:31;
+	__u32 pfmemalloc:1;
 #endif
 	/* we maintain a pagecount bias, so that we dont dirty cache line
 	 * containing page->_refcount every time we allocate a fragment.
 	 */
 	unsigned int		pagecnt_bias;
-	bool pfmemalloc;
 };
 
 static inline void page_frag_cache_init(struct page_frag_cache *nc)
@@ -43,7 +44,9 @@ static inline void *__page_frag_alloc_va_align(struct page_frag_cache *nc,
 					       gfp_t gfp_mask,
 					       unsigned int align)
 {
-	nc->offset = ALIGN(nc->offset, align);
+	unsigned int offset = nc->offset;
+
+	nc->offset = ALIGN(offset, align);
 
 	return page_frag_alloc_va(nc, fragsz, gfp_mask);
 }
@@ -53,7 +56,7 @@ static inline void *page_frag_alloc_va_align(struct page_frag_cache *nc,
 					     gfp_t gfp_mask,
 					     unsigned int align)
 {
-	WARN_ON_ONCE(!is_power_of_2(align));
+	WARN_ON_ONCE(!is_power_of_2(align) || align >= PAGE_SIZE);
 
 	return __page_frag_alloc_va_align(nc, fragsz, gfp_mask, align);
 }
diff --git a/mm/page_frag_alloc.c b/mm/page_frag_alloc.c
index 7f639af4e518..a02e57a439f0 100644
--- a/mm/page_frag_alloc.c
+++ b/mm/page_frag_alloc.c
@@ -29,7 +29,8 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
 		   __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
 	page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
 				PAGE_FRAG_CACHE_MAX_ORDER);
-	nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE;
+	nc->size_mask = page ? PAGE_FRAG_CACHE_MAX_SIZE - 1 : PAGE_SIZE - 1;
+	VM_BUG_ON(page && nc->size_mask != PAGE_FRAG_CACHE_MAX_SIZE - 1);
 #endif
 	if (unlikely(!page))
 		page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
@@ -88,7 +89,7 @@ void *page_frag_alloc_va(struct page_frag_cache *nc, unsigned int fragsz,
 
 #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
 	/* if size can vary use size else just use PAGE_SIZE */
-	size = nc->size;
+	size = nc->size_mask + 1;
 #else
 	size = PAGE_SIZE;
 #endif
-- 
2.33.0