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

On Tue, 11 Jan 2011, Daisuke Nishimura wrote:

quoted

What I recommend is below. (Please see the newest -mm because of a bug fix for
mem cgroup) Considering page management on radix-tree, it can be considerd as
a kind of page-migration, which replaces pages on radix-tree.

==

quoted

+int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
+{
+	int error;
+
+	VM_BUG_ON(!PageLocked(old));
+	VM_BUG_ON(!PageLocked(new));
+	VM_BUG_ON(new->mapping);
+

	struct mem_cgroup *memcg;

I think it should be initialized to NULL.

quoted

	error = mem_cgroup_prepare_migration(old, new, &memcg);

I want some comments like:

	/*
	 * This is not page migration, but prepare_migration and end_migration
	 * does enough work for charge replacement.
	 */

quoted

	#
	# This function will charge against "newpage". But this expects
	# the caller allows GFP_KERNEL gfp_mask. 
	# After this, the newpage is in "charged" state.
	if (error)
		return -ENOMEM;

quoted

+	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();

quoted

+		mem_cgroup_replace_cache_page(old, new); <== remove this.

		mem_cgroup_end_migraton(memcg, old, new, true);

quoted

+		page_cache_release(old);
+	} 

	else 
		mem_cgroup_end_migration(memcg, old, new, false);

	# Here, if the 4th argument is true, old page is uncharged.
	# if the 4th argument is false, the new page is uncharged.
	# Then, "charge" of the old page will be migrated onto the new page
	# if replacement is done.

quoted

+
+	return error;
+}
+EXPORT_SYMBOL_GPL(replace_page_cache_page);
+

==

I think this is enough simple and this covers all memory cgroup's racy
problems.

I agree.

Thanks for the comments.

Yeah, using existing infrastructure is undoubtedly simpler and less
prone to bugs.  So going with this for a first implementation might
do.

However, replace_page_cache_page() is meant to be very efficient,
otherwise any performance won by not copying the page contents are
lost to the cost of page replacement.  My guess is,
mem_cgroup_prepare_migration()/end_migration() are to heavyweight for
this.

Thanks,
Miklos

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