Re: [PATCH v4 1/6] mm: add AS_WRITEBACK_MAY_BLOCK mapping flag

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Nov 07, 2024 at 03:56:09PM -0800, Joanne Koong wrote:
> Add a new mapping flag AS_WRITEBACK_MAY_BLOCK which filesystems may set
> to indicate that writeback operations may block or take an indeterminate
> amount of time to complete. Extra caution should be taken when waiting
> on writeback for folios belonging to mappings where this flag is set.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  include/linux/pagemap.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 68a5f1ff3301..eb5a7837e142 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -210,6 +210,7 @@ enum mapping_flags {
>  	AS_STABLE_WRITES = 7,	/* must wait for writeback before modifying
>  				   folio contents */
>  	AS_INACCESSIBLE = 8,	/* Do not attempt direct R/W access to the mapping */
> +	AS_WRITEBACK_MAY_BLOCK = 9, /* Use caution when waiting on writeback */

To me 'may block' does not feel right. For example in reclaim code,
folio_wait_writeback() can get blocked and that is fine. However with
non-privileged fuse involved, there are security concerns. Somehow 'may
block' does not convey that. Anyways, I am not really pushing back but
I think there is a need for better name here.

>  	/* Bits 16-25 are used for FOLIO_ORDER */
>  	AS_FOLIO_ORDER_BITS = 5,
>  	AS_FOLIO_ORDER_MIN = 16,
> @@ -335,6 +336,16 @@ static inline bool mapping_inaccessible(struct address_space *mapping)
>  	return test_bit(AS_INACCESSIBLE, &mapping->flags);
>  }
>  
> +static inline void mapping_set_writeback_may_block(struct address_space *mapping)
> +{
> +	set_bit(AS_WRITEBACK_MAY_BLOCK, &mapping->flags);
> +}
> +
> +static inline bool mapping_writeback_may_block(struct address_space *mapping)
> +{
> +	return test_bit(AS_WRITEBACK_MAY_BLOCK, &mapping->flags);
> +}
> +
>  static inline gfp_t mapping_gfp_mask(struct address_space * mapping)
>  {
>  	return mapping->gfp_mask;
> -- 
> 2.43.5
>