On Tue, 18 Jan 2011 15:28:44 -0800
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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?
>
Poke?
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