Re: set_page_dirty races (was: Re: [patch 2/4] vfs: add set_page_dirty_notag)

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2009-02-17 at 12:25 +0100, Nick Piggin wrote:

> Introduce put_user_pages function.
> 
> In order to have more flexibility to deal with issues surrounding
> get_user_pages difficulties[*], introduce put_user_pages function
> intended to release pages acquired by get_user_pages. For now, just
> do the regular put_page thing. If all callers are converted, it could
> be used to help with such races. In the meantime, it will actually
> serve as a small extra piece of documentation for the code.
> 
> [*] eg. get_user_pages caller can bypass page_mkwrite calls into the
>     filesystem to notify of page dirty activity if the page gets cleaned
>     before the caller calls its final set_page_dirty).

Hmm, if we want to distinguish between .write=1 and .write=0, we would
have to pass .write to pup too, right?

> ---
>  include/linux/mm.h |    1 +
>  mm/memory.c        |   13 ++++++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6/include/linux/mm.h
> ===================================================================
> --- linux-2.6.orig/include/linux/mm.h
> +++ linux-2.6/include/linux/mm.h
> @@ -826,6 +826,7 @@ extern int access_process_vm(struct task
>  int get_user_pages(struct task_struct *tsk, struct mm_struct *mm, unsigned long start,
>  		int len, int write, int force, struct page **pages, struct vm_area_struct **vmas);
>  
> +void put_user_pages(struct page **pages, int nr);
>  extern int try_to_release_page(struct page * page, gfp_t gfp_mask);
>  extern void do_invalidatepage(struct page *page, unsigned long offset);
>  
> Index: linux-2.6/mm/memory.c
> ===================================================================
> --- linux-2.6.orig/mm/memory.c
> +++ linux-2.6/mm/memory.c
> @@ -1370,9 +1370,20 @@ int get_user_pages(struct task_struct *t
>  				start, len, flags,
>  				pages, vmas);
>  }
> -
>  EXPORT_SYMBOL(get_user_pages);
>  
> +/*
> + * put_user_pages should be used to release pages acquired with get_user_pages.
> + */
> +void put_user_pages(struct page **pages, int nr)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr; i++)
> +		put_page(pages[i]);
> +}
> +EXPORT_SYMBOL(put_user_pages);
> +
>  pte_t *get_locked_pte(struct mm_struct *mm, unsigned long addr,
>  			spinlock_t **ptl)
>  {
>