Re: [RFC 3/4] block: set mapping order for the block cache in set_init_blocksize

[Hannes Reinecke]

On 6/21/23 12:42, Pankaj Raghav wrote:
> > >        bdev->bd_inode->i_blkbits = blksize_bits(bsize);
> > > +    order = bdev->bd_inode->i_blkbits - PAGE_SHIFT;
> > > +    folio_order = mapping_min_folio_order(bdev->bd_inode->i_mapping);
> > > +
> > > +    if (!IS_ENABLED(CONFIG_BUFFER_HEAD)) {
> > > +        /* Do not allow changing the folio order after it is set */
> > > +        WARN_ON_ONCE(folio_order && (folio_order != order));
> > > +        mapping_set_folio_orders(bdev->bd_inode->i_mapping, order, 31);
> > > +    }
> > >    }
> > >      int set_blocksize(struct block_device *bdev, int size)
> > This really has nothing to do with buffer heads.
> >
> > In fact, I've got a patchset to make it work _with_ buffer heads.
> >
> > So please, don't make it conditional on CONFIG_BUFFER_HEAD.
> >
> > And we should be calling into 'mapping_set_folio_order()' only if the 'order' argument is larger
> > than PAGE_ORDER, otherwise we end up enabling
> > large folio support for _every_ block device.
> > Which I doubt we want.
>
> Hmm, which aops are you using for the block device? If you are using the old aops, then we will be
> using helpers from buffer.c and mpage.c which do not support large folios. I am getting a BUG_ON
> when I don't use iomap based aops for the block device:
I know. I haven't said that mpage.c / buffer.c support large folios 
_now_. All I'm saying is that I have a patchset enabling it to support 
large folios :-)

Cheers,

Hannes