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Date: Thu, 16 Dec 2010 10:07:44 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: add replace_page_cache_page() function
On Wed, 15 Dec 2010 16:49:58 +0100
Miklos Szeredi <miklos@...redi.hu> wrote:
> From: Miklos Szeredi <mszeredi@...e.cz>
>
> 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@...e.cz>
> ---
> 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);
>
> +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);
Hmm, then, the page will be recharged to "current" instead of the memcg
where "old" was under control. Is this design ? If so, why ?
In mm/migrate.c, following is called.
charge = mem_cgroup_prepare_migration(page, newpage, &mem);
....do migration....
if (!charge)
mem_cgroup_end_migration(mem, page, newpage);
BTW, off topic, in fuse/dev.c
add_to_page_cache_locked(page)
is called and this page is "charged" to memory cgroup. But, IIUC, this page
will be never be on LRU and cannot be reclaimed by memory cgroup.
I think this looks like a memory leak at rmdir() of memory cgroup and
rmdir will fail wish -EBUSY always.
So, I'd like to change this call something like as
add_to_page_cache_locked_and_no_memory_cgroup_control().
So, I think just dropping this memory cgroup related code is okay for us
because this is a replacement for add_to_page_cache_locked() which seems
problematic.
This will put pages on fuse's private radix-tree out of control.
Or, is it possible to drain these radix-tree pages at rmdir() of memory
cgroup by some call ?
Thanks,
-Kame
> + 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);
> + } 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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