The attached patch adds a function by which an existing page in the pagecache
may be traded for a new one at the same location without having to allocate or
free any radix tree nodes.
This permits CacheFS to write to start making a new version of a disk block in
memory for which the old version has not yet been written and journalled.
Signed-Off-By: David Howells <dhowells@xxxxxxxxxx>
---
warthog>diffstat -p1 replace-in-pagecache-2614mm2.diff
include/linux/pagemap.h | 3 +++
mm/filemap.c | 33 +++++++++++++++++++++++++++++++++
2 files changed, 36 insertions(+)
diff -uNrp linux-2.6.14-mm2/include/linux/pagemap.h linux-2.6.14-mm2-cachefs/include/linux/pagemap.h
--- linux-2.6.14-mm2/include/linux/pagemap.h 2005-11-14 16:17:59.000000000 +0000
+++ linux-2.6.14-mm2-cachefs/include/linux/pagemap.h 2005-11-14 16:24:47.000000000 +0000
@@ -96,6 +96,9 @@ int add_to_page_cache(struct page *page,
unsigned long index, gfp_t gfp_mask);
int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
unsigned long index, gfp_t gfp_mask);
+extern struct page *replace_in_page_cache(struct page *page,
+ struct address_space *mapping,
+ pgoff_t offset);
extern void remove_from_page_cache(struct page *page);
extern void __remove_from_page_cache(struct page *page);
diff -uNrp linux-2.6.14-mm2/mm/filemap.c linux-2.6.14-mm2-cachefs/mm/filemap.c
--- linux-2.6.14-mm2/mm/filemap.c 2005-11-14 16:18:00.000000000 +0000
+++ linux-2.6.14-mm2-cachefs/mm/filemap.c 2005-11-14 16:23:41.000000000 +0000
@@ -427,6 +427,39 @@ int add_to_page_cache_lru(struct page *p
EXPORT_SYMBOL(add_to_page_cache_lru);
/*
+ * This function replaces a page already in the page cache for a particular
+ * index with another, but only if there is already such a page in the page
+ * cache
+ */
+struct page *replace_in_page_cache(struct page *page,
+ struct address_space *mapping,
+ pgoff_t offset)
+{
+ struct page *old;
+ void **slot;
+
+ write_lock_irq(&mapping->tree_lock);
+
+ slot = radix_tree_lookup_slot(&mapping->page_tree, offset);
+ old = NULL;
+ if (slot) {
+ old = *slot;
+ *slot = page;
+ page_cache_get(page);
+ SetPageLocked(page);
+ page->mapping = mapping;
+ page->index = offset;
+ if (old)
+ old->mapping = NULL;
+ }
+
+ write_unlock_irq(&mapping->tree_lock);
+ return old;
+}
+
+EXPORT_SYMBOL(replace_in_page_cache);
+
+/*
* In order to wait for pages to become available there must be
* waitqueues associated with pages. By using a hash table of
* waitqueues where the bucket discipline is to maintain all
