[RFC PATCH v1 04/57] mm/page_alloc: Make page_frag_cache boot-time page size compatible

[Ryan Roberts <ryan.roberts@arm.com>]

"struct page_frag_cache" has some optimizations that depend on page
size. Let's refactor it a bit so that those optimizations can be
determined at run-time for the case where page size is a boot-time
parameter. For compile-time page size, the compiler should dead code
strip and the result is very similar to before.

One wrinkle is that we don't know if we need the size member until
runtime. So remove the ifdeffery and always define offset as u32 (needed
if PAGE_SIZE is >= 64K) and size as u16 (only used when PAGE_SIZE <=
32K). We move the members around a bit so that the overall size of the
struct remains the same; 24 bytes for 64-bit and 16 bytes on 32 bit.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

page_alloc
---

***NOTE***
Any confused maintainers may want to read the cover note here for context:
https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/

 include/linux/mm_types.h | 13 ++++++-------
 mm/page_alloc.c          | 31 ++++++++++++++++++-------------
 2 files changed, 24 insertions(+), 20 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 4854249792545..0844ed7cfaa53 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -544,16 +544,15 @@ static inline void *folio_get_private(struct folio *folio)
 
 struct page_frag_cache {
 	void * va;
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
-	__u16 offset;
-	__u16 size;
-#else
-	__u32 offset;
-#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;
+	unsigned int pagecnt_bias;
+	__u32 offset;
+	/* size only used when PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE, in which
+	 * case PAGE_FRAG_CACHE_MAX_SIZE is 32K and 16 bits is sufficient.
+	 */
+	__u16 size;
 	bool pfmemalloc;
 };
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 91ace8ca97e21..8678103b1b396 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4822,13 +4822,18 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
 	struct page *page = NULL;
 	gfp_t gfp = gfp_mask;
 
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
-	gfp_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) |  __GFP_COMP |
-		   __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;
-#endif
+	if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) {
+		gfp_mask = (gfp_mask & ~__GFP_DIRECT_RECLAIM) |  __GFP_COMP |
+			   __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC;
+		page = alloc_pages_node(NUMA_NO_NODE, gfp_mask,
+					PAGE_FRAG_CACHE_MAX_ORDER);
+		/*
+		 * Cast to silence warning due to 16-bit nc->size. Not real
+		 * because PAGE_SIZE only less than PAGE_FRAG_CACHE_MAX_SIZE
+		 * when PAGE_FRAG_CACHE_MAX_SIZE is 32K.
+		 */
+		nc->size = (__u16)(page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE);
+	}
 	if (unlikely(!page))
 		page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
 
@@ -4870,10 +4875,10 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
 		if (!page)
 			return NULL;
 
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
 		/* if size can vary use size else just use PAGE_SIZE */
-		size = nc->size;
-#endif
+		if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
+			size = nc->size;
+
 		/* Even if we own the page, we do not use atomic_set().
 		 * This would break get_page_unless_zero() users.
 		 */
@@ -4897,10 +4902,10 @@ void *__page_frag_alloc_align(struct page_frag_cache *nc,
 			goto refill;
 		}
 
-#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
 		/* if size can vary use size else just use PAGE_SIZE */
-		size = nc->size;
-#endif
+		if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
+			size = nc->size;
+
 		/* OK, page count is 0, we can safely set it */
 		set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1);
 
-- 
2.43.0