Re: [PATCH 1/5] brd: convert to folios

[Matthew Wilcox <willy@infradead.org>]

On Tue, Mar 07, 2023 at 09:14:27AM +0100, Hannes Reinecke wrote:
> On 3/7/23 08:30, Matthew Wilcox wrote:
> > On Tue, Mar 07, 2023 at 07:55:32AM +0100, Hannes Reinecke wrote:
> > > On 3/6/23 18:37, Matthew Wilcox wrote:
> > > > On Mon, Mar 06, 2023 at 01:01:23PM +0100, Hannes Reinecke wrote:
> > > > > -	page = alloc_page(gfp | __GFP_ZERO | __GFP_HIGHMEM);
> > > > > -	if (!page)
> > > > > +	folio = folio_alloc(gfp | __GFP_ZERO, 0);
> > > > > +	if (!folio)
> > > > 
> > > > Did you drop HIGHMEM support on purpose?
> > > 
> > > No; I thought that folios would be doing that implicitely.
> > > Will be re-adding.
> > 
> > We can't ... not all filesystems want to allocate every folio from
> > HIGHMEM.  eg for superblocks, it often makes more sense to allocate the
> > folio from lowmem than allocate it from highmem and keep it kmapped.
> > The only GFP flag that folios force-set is __GFP_COMP because folios by
> > definition are compound pages.
> 
> Oh well.
> 
> However, when playing with the modified brd and setting the logical&physical
> blocksize to 16k the whole thing crashes
> (not unexpectedly).
> It does crash, however, in block_read_full_folio(), which rather
> surprisingly (at least for me) is using create_page_buffers();
> I would have expected something like create_folio_buffers().
> Is this work in progress or what is the plan here?

Supporting folios > PAGE_SIZE in blockdev is definitely still WIP.
I know of at least one bug, which is:

#define bh_offset(bh)           ((unsigned long)(bh)->b_data & ~PAGE_MASK)

That needs to be something like

static size_t bh_offset(const struct buffer_head *bh)
{
	return (unsigned long)bh->b_data & (folio_size(bh->b_folio) - 1);
}

I haven't done a thorough scan for folio-size problems in the block
layer; I've just been fixing up things as I notice them.

Yes, create_page_buffers() should now be create_folio_buffers().  Just
didn't get round to it yet.