Re: [PATCH v3 02/11] fs: Allow fine-grained control of folio sizes

[Matthew Wilcox]

On Wed, Mar 13, 2024 at 06:02:44PM +0100, Pankaj Raghav (Samsung) wrote:
> +/*
> + * mapping_set_folio_min_order() - Set the minimum folio order
> + * @mapping: The address_space.
> + * @min: Minimum folio order (between 0-MAX_PAGECACHE_ORDER inclusive).
> + *
> + * The filesystem should call this function in its inode constructor to
> + * indicate which base size of folio the VFS can use to cache the contents
> + * of the file.  This should only be used if the filesystem needs special
> + * handling of folio sizes (ie there is something the core cannot know).
> + * Do not tune it based on, eg, i_size.
> + *
> + * Context: This should not be called while the inode is active as it
> + * is non-atomic.
> + */
> +static inline void mapping_set_folio_min_order(struct address_space *mapping,
> +					       unsigned int min)
> +{
> +	if (min > MAX_PAGECACHE_ORDER)
> +		min = MAX_PAGECACHE_ORDER;
> +
> +	mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
> +			 (min << AS_FOLIO_ORDER_MIN) |
> +			 (MAX_PAGECACHE_ORDER << AS_FOLIO_ORDER_MAX);
> +}

I was surprised when I read this, which indicates it might be surprising
for others too.  I think it at least needs a comment to say that the
maximum will be set to the MAX_PAGECACHE_ORDER, because I was expecting
it to set max == min.  I guess that isn't what XFS wants, but someone
doing this to, eg, ext4 is going to have an unpleasant surprise when
they call into block_read_full_folio() and overrun 'arr'.

I'm still not entirely convinced this wouldn't be better to do as
mapping_set_folio_order_range() and have

static inline void mapping_set_folio_min_order(struct address_space *mapping,
		unsigned int min)
{
	mapping_set_folio_range(mapping, min, MAX_PAGECACHE_ORDER);
}