[RFC PATCH 1/3] Supporting hacks to be able to test slab allocated buffers in place of page_frag without rewriting lots of net code. We make several assumptions here, first that slab allocator is selected. Second, no one is doing get_page or put_page on pages marked PG_slab. Third we allocated all slabs page aligned that we do these calls on.

[Debabrata Banerjee <dbanerje@akamai.com>]

---
 include/linux/mm.h |  6 ++++++
 mm/slab.c          |  8 ++++++++
 mm/swap.c          | 13 ++++++++++++-
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e0c8528..de21a92 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -398,12 +398,18 @@ static inline void get_huge_page_tail(struct page *page)
 }
 
 extern bool __get_page_tail(struct page *page);
+extern struct page *slabpage_to_headpage(struct page *page);
 
 static inline void get_page(struct page *page)
 {
 	if (unlikely(PageTail(page)))
 		if (likely(__get_page_tail(page)))
 			return;
+
+	//Hack for slab page
+	if (unlikely(page->flags & (1L << PG_slab)))
+		page = slabpage_to_headpage(page);
+
 	/*
 	 * Getting a normal page or the head of a compound page
 	 * requires to already have an elevated page->_count.
diff --git a/mm/slab.c b/mm/slab.c
index bd88411..36d5176 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -483,6 +483,14 @@ static inline unsigned int obj_to_index(const struct kmem_cache *cache,
 	return reciprocal_divide(offset, cache->reciprocal_buffer_size);
 }
 
+struct page *slabpage_to_headpage(struct page *page)
+{
+	//Hack to support get_page/put_page on slabs bigger than a page
+	unsigned int idx = obj_to_index(page->slab_cache, page->slab_page, page_address(page));
+	return virt_to_page(index_to_obj(page->slab_cache, page->slab_page, idx));
+}
+EXPORT_SYMBOL(slabpage_to_headpage);
+
 static struct arraycache_init initarray_generic =
     { {0, BOOT_CPUCACHE_ENTRIES, 1, 0} };
 
diff --git a/mm/swap.c b/mm/swap.c
index 9f2225f..94c75bc 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -172,9 +172,20 @@ skip_lock_tail:
 	}
 }
 
+extern struct page *slabpage_to_headpage(struct page *page);
+
 void put_page(struct page *page)
 {
-	if (unlikely(PageCompound(page)))
+	if (unlikely(page->flags & (1L << PG_slab))) {
+		struct page *head_page = slabpage_to_headpage(page);
+		//Hack. Assume we have >PAGE_SIZE and aligned slabs, and no one is dumb enough
+		//to do a put_page to 0 on a slab page without meaning to free it from the slab.
+		if (put_page_testzero(head_page)) {
+			get_page(head_page); //restore 1 _count for slab
+			kmem_cache_free(page->slab_cache, page_address(head_page));
+		}
+	}
+	else if (unlikely(PageCompound(page)))
 		put_compound_page(page);
 	else if (put_page_testzero(page))
 		__put_single_page(page);
-- 
1.8.3.4

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