[RFC][PATCH 2/7] make 'struct page' and swp_entry_t variants of swapcache_free().

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

swapcache_free() takes two arguments:

	void swapcache_free(swp_entry_t entry, struct page *page)

Most of its callers (5/7) are from error handling paths haven't even
instantiated a page, so they pass page=NULL.  Both of the callers
that call in with a 'struct page' create and pass in a temporary
swp_entry_t.

Now that we are deferring clearing page_private() until after
swapcache_free() has been called, we can just create a variant
that takes a 'struct page' and does the temporary variable in
the helper.

That leaves all the other callers doing

	swapcache_free(entry, NULL)

so create another helper for them that makes it clear that they
need only pass in a swp_entry_t.

One downside here is that delete_from_swap_cache() now does
an extra swap_address_space() call.  But, those are pretty
cheap (just some array index arithmetic).

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
---

 linux.git-davehans/drivers/staging/zcache/zcache-main.c |    2 +-
 linux.git-davehans/include/linux/swap.h                 |    3 ++-
 linux.git-davehans/mm/shmem.c                           |    2 +-
 linux.git-davehans/mm/swap_state.c                      |   13 +++++--------
 linux.git-davehans/mm/swapfile.c                        |   13 ++++++++++++-
 linux.git-davehans/mm/vmscan.c                          |    3 +--
 6 files changed, 22 insertions(+), 14 deletions(-)

diff -puN drivers/staging/zcache/zcache-main.c~make-page-and-swp_entry_t-variants drivers/staging/zcache/zcache-main.c
--- linux.git/drivers/staging/zcache/zcache-main.c~make-page-and-swp_entry_t-variants	2013-05-07 13:48:13.963056205 -0700
+++ linux.git-davehans/drivers/staging/zcache/zcache-main.c	2013-05-07 13:48:13.975056737 -0700
@@ -961,7 +961,7 @@ static int zcache_get_swap_cache_page(in
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
-		swapcache_free(entry, NULL);
+		swapcache_free_entry(entry);
 		/* FIXME: is it possible to get here without err==-ENOMEM?
 		 * If not, we can dispense with the do loop, use goto retry */
 	} while (err != -ENOMEM);
diff -puN include/linux/swap.h~make-page-and-swp_entry_t-variants include/linux/swap.h
--- linux.git/include/linux/swap.h~make-page-and-swp_entry_t-variants	2013-05-07 13:48:13.964056249 -0700
+++ linux.git-davehans/include/linux/swap.h	2013-05-07 13:48:13.975056737 -0700
@@ -382,7 +382,8 @@ extern void swap_shmem_alloc(swp_entry_t
 extern int swap_duplicate(swp_entry_t);
 extern int swapcache_prepare(swp_entry_t);
 extern void swap_free(swp_entry_t);
-extern void swapcache_free(swp_entry_t, struct page *page);
+extern void swapcache_free_entry(swp_entry_t entry);
+extern void swapcache_free_page_entry(struct page *page);
 extern int free_swap_and_cache(swp_entry_t);
 extern int swap_type_of(dev_t, sector_t, struct block_device **);
 extern unsigned int count_swap_pages(int, int);
diff -puN mm/shmem.c~make-page-and-swp_entry_t-variants mm/shmem.c
--- linux.git/mm/shmem.c~make-page-and-swp_entry_t-variants	2013-05-07 13:48:13.966056339 -0700
+++ linux.git-davehans/mm/shmem.c	2013-05-07 13:48:13.976056781 -0700
@@ -871,7 +871,7 @@ static int shmem_writepage(struct page *
 	}
 
 	mutex_unlock(&shmem_swaplist_mutex);
-	swapcache_free(swap, NULL);
+	swapcache_free_entry(swap);
 redirty:
 	set_page_dirty(page);
 	if (wbc->for_reclaim)
diff -puN mm/swapfile.c~make-page-and-swp_entry_t-variants mm/swapfile.c
--- linux.git/mm/swapfile.c~make-page-and-swp_entry_t-variants	2013-05-07 13:48:13.968056427 -0700
+++ linux.git-davehans/mm/swapfile.c	2013-05-07 13:48:13.977056825 -0700
@@ -637,7 +637,7 @@ void swap_free(swp_entry_t entry)
 /*
  * Called after dropping swapcache to decrease refcnt to swap entries.
  */
-void swapcache_free(swp_entry_t entry, struct page *page)
+static void __swapcache_free(swp_entry_t entry, struct page *page)
 {
 	struct swap_info_struct *p;
 	unsigned char count;
@@ -651,6 +651,17 @@ void swapcache_free(swp_entry_t entry, s
 	}
 }
 
+void swapcache_free_entry(swp_entry_t entry)
+{
+	__swapcache_free(entry, NULL);
+}
+
+void swapcache_free_page_entry(struct page *page)
+{
+	swp_entry_t entry = { .val = page_private(page) };
+	__swapcache_free(entry, page);
+}
+
 /*
  * How many references to page are currently swapped out?
  * This does not give an exact answer when swap count is continued,
diff -puN mm/swap_state.c~make-page-and-swp_entry_t-variants mm/swap_state.c
--- linux.git/mm/swap_state.c~make-page-and-swp_entry_t-variants	2013-05-07 13:48:13.969056471 -0700
+++ linux.git-davehans/mm/swap_state.c	2013-05-07 13:48:13.978056869 -0700
@@ -172,7 +172,7 @@ int add_to_swap(struct page *page, struc
 
 	if (unlikely(PageTransHuge(page)))
 		if (unlikely(split_huge_page_to_list(page, list))) {
-			swapcache_free(entry, NULL);
+			swapcache_free_entry(entry);
 			return 0;
 		}
 
@@ -198,7 +198,7 @@ int add_to_swap(struct page *page, struc
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
-		swapcache_free(entry, NULL);
+		swapcache_free_entry(entry);
 		return 0;
 	}
 }
@@ -211,17 +211,14 @@ int add_to_swap(struct page *page, struc
  */
 void delete_from_swap_cache(struct page *page)
 {
-	swp_entry_t entry;
 	struct address_space *address_space;
 
-	entry.val = page_private(page);
-
-	address_space = swap_address_space(entry);
+	address_space = page_mapping(page);
 	spin_lock_irq(&address_space->tree_lock);
 	__delete_from_swap_cache(page);
 	spin_unlock_irq(&address_space->tree_lock);
 
-	swapcache_free(entry, page);
+	swapcache_free_page_entry(page);
 	set_page_private(page, 0);
 	ClearPageSwapCache(page);
 	page_cache_release(page);
@@ -365,7 +362,7 @@ struct page *read_swap_cache_async(swp_e
 		 * add_to_swap_cache() doesn't return -EEXIST, so we can safely
 		 * clear SWAP_HAS_CACHE flag.
 		 */
-		swapcache_free(entry, NULL);
+		swapcache_free_entry(entry);
 	} while (err != -ENOMEM);
 
 	if (new_page)
diff -puN mm/vmscan.c~make-page-and-swp_entry_t-variants mm/vmscan.c
--- linux.git/mm/vmscan.c~make-page-and-swp_entry_t-variants	2013-05-07 13:48:13.971056559 -0700
+++ linux.git-davehans/mm/vmscan.c	2013-05-07 13:48:13.979056913 -0700
@@ -490,10 +490,9 @@ static int __remove_mapping(struct addre
 	}
 
 	if (PageSwapCache(page)) {
-		swp_entry_t swap = { .val = page_private(page) };
 		__delete_from_swap_cache(page);
 		spin_unlock_irq(&mapping->tree_lock);
-		swapcache_free(swap, page);
+		swapcache_free_page_entry(page);
 		set_page_private(page, 0);
 		ClearPageSwapCache(page);
 	} else {
_
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