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Subject[PATCH 24/29] mm: methods for teaching filesystems about PG_swapcache pages
In order to teach filesystems to handle swap cache pages, two new page
functions are introduced:

pgoff_t page_file_index(struct page *);
struct address_space *page_file_mapping(struct page *);

page_file_index - gives the offset of this page in the file in PAGE_CACHE_SIZE
blocks. Like page->index is for mapped pages, this function also gives the
correct index for PG_swapcache pages.

page_file_mapping - gives the mapping backing the actual page; that is for
swap cache pages it will give swap_file->f_mapping.

page_offset() is modified to use page_file_index(), so that it will give the
expected result, even for PG_swapcache pages.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/mm.h | 26 ++++++++++++++++++++++++++
include/linux/pagemap.h | 2 +-
2 files changed, 27 insertions(+), 1 deletion(-)

Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -14,6 +14,7 @@
#include <linux/mm_types.h>
#include <linux/security.h>
#include <linux/swap.h>
+#include <linux/fs.h>

struct mempolicy;
struct anon_vma;
@@ -608,6 +609,16 @@ static inline struct swap_info_struct *p
return get_swap_info_struct(swp_type(swap));
}

+static inline
+struct address_space *page_file_mapping(struct page *page)
+{
+#ifdef CONFIG_SWAP_FILE
+ if (unlikely(PageSwapCache(page)))
+ return page_swap_info(page)->swap_file->f_mapping;
+#endif
+ return page->mapping;
+}
+
static inline int PageAnon(struct page *page)
{
return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
@@ -625,6 +636,21 @@ static inline pgoff_t page_index(struct
}

/*
+ * Return the file index of the page. Regular pagecache pages use ->index
+ * whereas swapcache pages use swp_offset(->private)
+ */
+static inline pgoff_t page_file_index(struct page *page)
+{
+#ifdef CONFIG_SWAP_FILE
+ if (unlikely(PageSwapCache(page))) {
+ swp_entry_t swap = { .val = page_private(page) };
+ return swp_offset(swap);
+ }
+#endif
+ return page->index;
+}
+
+/*
* The atomic page->_mapcount, like _count, starts from -1:
* so that transitions both from it and to it can be tracked,
* using atomic_inc_and_test and atomic_add_negative(-1).
Index: linux-2.6/include/linux/pagemap.h
===================================================================
--- linux-2.6.orig/include/linux/pagemap.h
+++ linux-2.6/include/linux/pagemap.h
@@ -145,7 +145,7 @@ extern void __remove_from_page_cache(str
*/
static inline loff_t page_offset(struct page *page)
{
- return ((loff_t)page->index) << PAGE_CACHE_SHIFT;
+ return ((loff_t)page_file_index(page)) << PAGE_CACHE_SHIFT;
}

static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
--



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