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

[Minchan Kim]

On Thu, Dec 16, 2010 at 12:49 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> From: Miklos Szeredi <mszeredi@xxxxxxx>
>
> 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.

Please write down why fuse need this new atomic function in description.

>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
> ---
>  fs/fuse/dev.c           |   10 ++++------
>  include/linux/pagemap.h |    1 +
>  mm/filemap.c            |   41 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 46 insertions(+), 6 deletions(-)
>
> Index: linux-2.6/mm/filemap.c
> ===================================================================
> --- linux-2.6.orig/mm/filemap.c 2010-12-15 16:39:55.000000000 +0100
> +++ linux-2.6/mm/filemap.c      2010-12-15 16:41:24.000000000 +0100
> @@ -389,6 +389,47 @@ int filemap_write_and_wait_range(struct
>  }
>  EXPORT_SYMBOL(filemap_write_and_wait_range);
>

This function is exported.
Please, add function description

> +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);
> +
> +       error = mem_cgroup_cache_charge(new, current->mm,
> +                                       gfp_mask & GFP_RECLAIM_MASK);
> +       if (error)
> +               goto out;
> +
> +       error = radix_tree_preload(gfp_mask & ~__GFP_HIGHMEM);
> +       if (error == 0) {
> +               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();
> +               mem_cgroup_uncharge_cache_page(old);
> +               page_cache_release(old);

Why do you release reference of old?

> +       } else
> +               mem_cgroup_uncharge_cache_page(new);
> +out:
> +       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
> Index: linux-2.6/include/linux/pagemap.h
> ===================================================================
> --- linux-2.6.orig/include/linux/pagemap.h      2010-12-15 16:39:39.000000000 +0100
> +++ linux-2.6/include/linux/pagemap.h   2010-12-15 16:41:24.000000000 +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        2010-12-15 16:39:39.000000000 +0100
> +++ linux-2.6/fs/fuse/dev.c     2010-12-15 16:41:24.000000000 +0100
> @@ -729,14 +729,12 @@ 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))
>
> --
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-- 
Kind regards,
Minchan Kim
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