On Tue, 18 Jan 2011 12:18:11 +0100
Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> +int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
> +{
> + int error;
> + struct mem_cgroup *memcg = NULL;
I'm suspecting that the unneeded initialisation was added to suppress a
warning?
I removed it, and didn't get a warning. I expected to.
Really, uninitialized_var() is better. It avoids adding extra code
and, unlike "= 0" it is self-documenting.
> + 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);
This is all pretty ugly and inefficient.
We call __remove_from_page_cache() which does a radix-tree lookup and
then fiddles a bunch of accounting things.
Then we immediately do the same radix-tree lookup and then undo the
accounting changes which we just did. And we do it in an open-coded
fashion, thus giving the kernel yet another code site where various
operations need to be kept in sync.
Would it not be better to do a single radix_tree_lookup_slot(),
overwrite the pointer therein and just leave all the ancilliary
accounting unaltered?
> + } else {
> + mem_cgroup_end_migration(memcg, old, new, false);
> + }
> +
> + return error;
> +}
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