Re: [PATCH v2] fix __set_page_dirty_no_writeback() return value

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

On Thu, Nov 11, 2010 at 11:05:54AM +0800, Bob Liu wrote:
> __set_page_dirty_no_writeback() should return true if it actually transitioned
> the page from a clean to dirty state although it seems nobody used its return
> value now.
> 
> Change from v1:
> 	* preserving cacheline optimisation as Andrew pointed out
> 
> Signed-off-by: Bob Liu <lliubbo@gmail.com>
> ---
>  mm/page-writeback.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index bf85062..ac7018a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1157,8 +1157,10 @@ EXPORT_SYMBOL(write_one_page);
>   */
>  int __set_page_dirty_no_writeback(struct page *page)
>  {
> -	if (!PageDirty(page))
> +	if (!PageDirty(page)) {
>  		SetPageDirty(page);
> +		return 1;
> +	}
>  	return 0;
>  }

It's still racy if not using TestSetPageDirty(). In fact
set_page_dirty() has a default reference implementation:

        if (!PageDirty(page)) {
                if (!TestSetPageDirty(page))
                        return 1;
        }
        return 0;

It seems the return value currently is only tested for doing
balance_dirty_pages_ratelimited(). So not a big problem.

Thanks,
Fengguang

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