Re: [PATCH v3] mm: add replace_page_cache_page() function

[Miklos Szeredi]

On Wed, 12 Jan 2011, KAMEZAWA Hiroyuki wrote:
> On Tue, 11 Jan 2011 23:25:28 +0900
> Minchan Kim <minchan.kim@xxxxxxxxx> wrote:
> 
> > On Tue, Jan 11, 2011 at 03:07:54PM +0100, Miklos Szeredi wrote:
> > > (resent with fixed CC list, sorry for the duplicate)
> > > 
> > > Thanks for the review.
> > > 
> > > Here's an updated patch.  Modifications since the last post:
> > > 
> > >  - don't pass gfp_mask (since it's only able to deal with GFP_KERNEL
> > >    anyway)
> > > 
> > 
> > I am not sure it's a good idea.
> > Now if we need just GFP_KERNEL, we can't make sure it in future.
> > Sometime we might need GFP_ATOMIC and friendd functions
> > (ex, add_to_page_cache_lru,add_to_page_cache_locked) already have gfp_mask.
> > It's a exported function so it's hard to modify it in future.
> > 
> > I want to keep it.
> > Instead of removing it, we can change mem_cgroup_prepare_migration as
> > getting gfp_mask.
> > 
> you're right.

Okay, makes sense.

Here's an updated patch.

Thanks,
Miklos
---

From: Miklos Szeredi <mszeredi@xxxxxxx>
Subject: mm: add replace_page_cache_page() function

This function basically does:

     remove_from_page_cache(old);
     page_cache_release(old);
     add_to_page_cache_locked(new);

Except it does this atomically, so there's no possibility for the
"add" to fail because of a race.

This is used by fuse to move pages into the page cache.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---
 fs/fuse/dev.c              |   10 ++----
 include/linux/memcontrol.h |    4 +-
 include/linux/pagemap.h    |    1 
 mm/filemap.c               |   65 +++++++++++++++++++++++++++++++++++++++++++++
 mm/memcontrol.c            |    4 +-
 5 files changed, 74 insertions(+), 10 deletions(-)

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c	2011-01-12 14:50:18.000000000 +0100
+++ linux-2.6/mm/filemap.c	2011-01-12 14:51:25.000000000 +0100
@@ -387,6 +387,71 @@ int filemap_write_and_wait_range(struct
 EXPORT_SYMBOL(filemap_write_and_wait_range);
 
 /**
+ * replace_page_cache_page - replace a pagecache page with a new one
+ * @old:	page to be replaced
+ * @new:	page to replace with
+ * @gfp_mask:	allocation mode
+ *
+ * This function replaces a page in the pagecache with a new one.  On
+ * success it acquires the pagecache reference for the new page and
+ * drops it for the old page.  Both the old and new pages must be
+ * locked.  This function does not add the new page to the LRU, the
+ * caller must do that.
+ *
+ * The remove + add is atomic.  The only way this function can fail is
+ * memory allocation failure.
+ */
+int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
+{
+	int error;
+	struct mem_cgroup *memcg = NULL;
+
+	VM_BUG_ON(!PageLocked(old));
+	VM_BUG_ON(!PageLocked(new));
+	VM_BUG_ON(new->mapping);
+
+	/*
+	 * This is not page migration, but prepare_migration and
+	 * end_migration does enough work for charge replacement.
+	 *
+	 * In the longer term we probably want a specialized function
+	 * for moving the charge from old to new in a more efficient
+	 * manner.
+	 */
+	error = mem_cgroup_prepare_migration(old, new, &memcg, gfp_mask);
+	if (error)
+		return error;
+
+	error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
+	if (!error) {
+		struct address_space *mapping = old->mapping;
+		pgoff_t offset = old->index;
+
+		page_cache_get(new);
+		new->mapping = mapping;
+		new->index = offset;
+
+		spin_lock_irq(&mapping->tree_lock);
+		__remove_from_page_cache(old);
+		error = radix_tree_insert(&mapping->page_tree, offset, new);
+		BUG_ON(error);
+		mapping->nrpages++;
+		__inc_zone_page_state(new, NR_FILE_PAGES);
+		if (PageSwapBacked(new))
+			__inc_zone_page_state(new, NR_SHMEM);
+		spin_unlock_irq(&mapping->tree_lock);
+		radix_tree_preload_end();
+		page_cache_release(old);
+		mem_cgroup_end_migration(memcg, old, new, true);
+	} else {
+		mem_cgroup_end_migration(memcg, old, new, false);
+	}
+
+	return error;
+}
+EXPORT_SYMBOL_GPL(replace_page_cache_page);
+
+/**
  * add_to_page_cache_locked - add a locked page to the pagecache
  * @page:	page to add
  * @mapping:	the page's address_space
Index: linux-2.6/include/linux/pagemap.h
===================================================================
--- linux-2.6.orig/include/linux/pagemap.h	2011-01-12 14:50:18.000000000 +0100
+++ linux-2.6/include/linux/pagemap.h	2011-01-12 14:50:31.000000000 +0100
@@ -457,6 +457,7 @@ int add_to_page_cache_lru(struct page *p
 				pgoff_t index, gfp_t gfp_mask);
 extern void remove_from_page_cache(struct page *page);
 extern void __remove_from_page_cache(struct page *page);
+int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask);
 
 /*
  * Like add_to_page_cache_locked, but used to add newly allocated pages:
Index: linux-2.6/fs/fuse/dev.c
===================================================================
--- linux-2.6.orig/fs/fuse/dev.c	2011-01-12 14:50:18.000000000 +0100
+++ linux-2.6/fs/fuse/dev.c	2011-01-12 14:50:31.000000000 +0100
@@ -737,14 +737,12 @@ static int fuse_try_move_page(struct fus
 	if (WARN_ON(PageMlocked(oldpage)))
 		goto out_fallback_unlock;
 
-	remove_from_page_cache(oldpage);
-	page_cache_release(oldpage);
-
-	err = add_to_page_cache_locked(newpage, mapping, index, GFP_KERNEL);
+	err = replace_page_cache_page(oldpage, newpage, GFP_KERNEL);
 	if (err) {
-		printk(KERN_WARNING "fuse_try_move_page: failed to add page");
-		goto out_fallback_unlock;
+		unlock_page(newpage);
+		return err;
 	}
+
 	page_cache_get(newpage);
 
 	if (!(buf->flags & PIPE_BUF_FLAG_LRU))
Index: linux-2.6/include/linux/memcontrol.h
===================================================================
--- linux-2.6.orig/include/linux/memcontrol.h	2011-01-12 14:50:19.000000000 +0100
+++ linux-2.6/include/linux/memcontrol.h	2011-01-12 14:53:13.000000000 +0100
@@ -91,7 +91,7 @@ extern struct cgroup_subsys_state *mem_c
 
 extern int
 mem_cgroup_prepare_migration(struct page *page,
-	struct page *newpage, struct mem_cgroup **ptr);
+	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask);
 extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
 	struct page *oldpage, struct page *newpage, bool migration_ok);
 
@@ -225,7 +225,7 @@ static inline struct cgroup_subsys_state
 
 static inline int
 mem_cgroup_prepare_migration(struct page *page, struct page *newpage,
-	struct mem_cgroup **ptr)
+	struct mem_cgroup **ptr, gfp_t gfp_mask)
 {
 	return 0;
 }
Index: linux-2.6/mm/memcontrol.c
===================================================================
--- linux-2.6.orig/mm/memcontrol.c	2011-01-12 14:50:19.000000000 +0100
+++ linux-2.6/mm/memcontrol.c	2011-01-12 14:50:39.000000000 +0100
@@ -2767,7 +2767,7 @@ static inline int mem_cgroup_move_swap_a
  * page belongs to.
  */
 int mem_cgroup_prepare_migration(struct page *page,
-	struct page *newpage, struct mem_cgroup **ptr)
+	struct page *newpage, struct mem_cgroup **ptr, gfp_t gfp_mask)
 {
 	struct page_cgroup *pc;
 	struct mem_cgroup *mem = NULL;
@@ -2823,7 +2823,7 @@ int mem_cgroup_prepare_migration(struct
 		return 0;
 
 	*ptr = mem;
-	ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, ptr, false);
+	ret = __mem_cgroup_try_charge(NULL, gfp_mask, ptr, false);
 	css_put(&mem->css);/* drop extra refcnt */
 	if (ret || *ptr == NULL) {
 		if (PageAnon(page)) {
--
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