Re: [PATCH 1/3] mm: helper functions for dirty and writeback accounting

[Wu Fengguang <fengguang.wu@intel.com>]

On Thu, Aug 19, 2010 at 01:57:25PM -0700, Michael Rubin wrote:
> Exporting account_pages_dirty and adding a symmetric routine
> account_pages_writeback.

s/account_pages_writeback/account_page_writeback/

I'd recommend to separate the changes into two patches.
It's actually a bug fix to export account_pages_dirty() for ceph,
which should be a good candidate for 2.6.36.

> This allows code outside of the mm core to safely manipulate page state
> and not worry about the other accounting. Not using these routines means
> that some code will lose track of the accounting and we get bugs. This
> has happened once already.
> 
> Signed-off-by: Michael Rubin <mrubin@google.com>
> ---
>  fs/ceph/addr.c      |    8 ++------
>  fs/nilfs2/segment.c |    2 +-
>  include/linux/mm.h  |    1 +
>  mm/page-writeback.c |   15 +++++++++++++++
>  4 files changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index d9c60b8..359aa3a 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -106,12 +106,8 @@ static int ceph_set_page_dirty(struct page *page)
>  	if (page->mapping) {	/* Race with truncate? */
>  		WARN_ON_ONCE(!PageUptodate(page));
>  
> -		if (mapping_cap_account_dirty(mapping)) {
> -			__inc_zone_page_state(page, NR_FILE_DIRTY);
> -			__inc_bdi_stat(mapping->backing_dev_info,
> -					BDI_RECLAIMABLE);
> -			task_io_account_write(PAGE_CACHE_SIZE);
> -		}
> +		if (mapping_cap_account_dirty(mapping))

That 'if' is not necessary. account_page_dirtied() already has one.
The extra 'if' is not an optimization either, because the ceph fs is
not likely to have un-accountable mappings.

> +			account_page_dirtied(page, page->mapping);
>  		radix_tree_tag_set(&mapping->page_tree,
>  				page_index(page), PAGECACHE_TAG_DIRTY);
>  
> diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
> index c920164..967ed7d 100644
> --- a/fs/nilfs2/segment.c
> +++ b/fs/nilfs2/segment.c
> @@ -1599,7 +1599,7 @@ nilfs_copy_replace_page_buffers(struct page *page, struct list_head *out)
>  	kunmap_atomic(kaddr, KM_USER0);
>  
>  	if (!TestSetPageWriteback(clone_page))
> -		inc_zone_page_state(clone_page, NR_WRITEBACK);
> +		account_page_writeback(clone_page, page_mapping(clone_page));
>  	unlock_page(clone_page);
>  
>  	return 0;
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index a2b4804..b138392 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -855,6 +855,7 @@ int __set_page_dirty_no_writeback(struct page *page);
>  int redirty_page_for_writepage(struct writeback_control *wbc,
>  				struct page *page);
>  void account_page_dirtied(struct page *page, struct address_space *mapping);
> +void account_page_writeback(struct page *page, struct address_space *mapping);
>  int set_page_dirty(struct page *page);
>  int set_page_dirty_lock(struct page *page);
>  int clear_page_dirty_for_io(struct page *page);
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 37498ef..b8e7b3b 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1096,6 +1096,21 @@ void account_page_dirtied(struct page *page, struct address_space *mapping)
>  		task_io_account_write(PAGE_CACHE_SIZE);
>  	}
>  }
> +EXPORT_SYMBOL(account_page_dirtied);
> +
> +/*
> + * Helper function for set_page_writeback family.
> + * NOTE: Unlike account_page_dirtied this does not rely on being atomic
> + * wrt interrupts.
> + */
> +
> +void account_page_writeback(struct page *page, struct address_space *mapping)
> +{
> +	if (mapping_cap_account_dirty(mapping))

The 'if' test and *mapping parameter looks unnecessary at least for
now. The only place a mapping has BDI_CAP_NO_ACCT_WB but not
BDI_CAP_NO_WRITEBACK is fuse, which does its own accounting.

Thanks,
Fengguang

> +		inc_zone_page_state(page, NR_WRITEBACK);
> +}
> +EXPORT_SYMBOL(account_page_writeback);
> +
>  
>  /*
>   * For address_spaces which do not use buffers.  Just tag the page as dirty in
> -- 
> 1.7.1
> 
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