Re: [PATCH 12/20] mm: Factor out common parts of write fault handling

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Tue, Sep 27, 2016 at 06:08:16PM +0200, Jan Kara wrote:
> Currently we duplicate handling of shared write faults in
> wp_page_reuse() and do_shared_fault(). Factor them out into a common
> function.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/memory.c | 78 +++++++++++++++++++++++++++++--------------------------------
>  1 file changed, 37 insertions(+), 41 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 63d9c1a54caf..0643b3b5a12a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2063,6 +2063,41 @@ static int do_page_mkwrite(struct vm_area_struct *vma, struct page *page,
>  }
>  
>  /*
> + * Handle dirtying of a page in shared file mapping on a write fault.
> + *
> + * The function expects the page to be locked and unlocks it.
> + */
> +static void fault_dirty_shared_page(struct vm_area_struct *vma,
> +				    struct page *page)
> +{
> +	struct address_space *mapping;
> +	bool dirtied;
> +	bool page_mkwrite = vma->vm_ops->page_mkwrite;

I think you may need to pass in a 'page_mkwrite' parameter if you don't want
to change behavior.  Just checking to see of vma->vm_ops->page_mkwrite is
non-NULL works fine for this path:

do_shared_fault()
	fault_dirty_shared_page()

and for

wp_page_shared()
	wp_page_reuse()
		fault_dirty_shared_page()

But for these paths:

wp_pfn_shared()
	wp_page_reuse()
		fault_dirty_shared_page()

and

do_wp_page()
	wp_page_reuse()
		fault_dirty_shared_page()

we unconditionally pass 0 for the 'page_mkwrite' parameter, even though from
the logic in wp_pfn_shared() especially you can see that
vma->vm_ops->pfn_mkwrite() must be defined some of the time.

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