On Mon, Mar 25, 2024 at 06:29:30PM +0000, Matthew Wilcox wrote:
> 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);
> }
I agree. Having a helper like this will make it more explicit. The
limits checking can also be done in this helper itself.
Also it makes mapping_set_large_folio() more clear:
static inline void mapping_set_large_folios(struct address_space *mapping)
{
mapping_set_folio_range(mapping, 0, MAX_PAGECACHE_ORDER);
}
instead of just calling mapping_set_folio_min_order(). Thanks.
