Re: [PATCH 1/6] filemap: make the folio order calculation shareable

[Luis Chamberlain <mcgrof@kernel.org>]

On Fri, Sep 15, 2023 at 02:40:07PM +0100, Matthew Wilcox wrote:
> On Fri, Sep 15, 2023 at 09:51:23AM +0000, Daniel Gomez wrote:
> > To make the code that clamps the folio order in the __filemap_get_folio
> > routine reusable to others, move and merge it to the fgf_set_order
> > new subroutine (mapping_size_order), so when mapping the size at a
> > given index, the order calculated is already valid and ready to be
> > used when order is retrieved from fgp_flags with FGF_GET_ORDER.
> > 
> > Signed-off-by: Daniel Gomez <da.gomez@samsung.com>
> > ---
> >  fs/iomap/buffered-io.c  |  6 ++++--
> >  include/linux/pagemap.h | 42 ++++++++++++++++++++++++++++++++++++-----
> >  mm/filemap.c            |  8 --------
> >  3 files changed, 41 insertions(+), 15 deletions(-)
> 
> That seems like a lot of extra code to add in order to avoid copying
> six lines of code and one comment into the shmem code.
> 
> It's not wrong, but it seems like a bad tradeoff to me.

The suggestion to merge came from me, mostly based on later observations
that in the future we may want to extend this with a min order to ensure
the index is aligned the the order. This check would only be useful for
buffred IO for iomap, readahead. It has me wondering if buffer-heads
support for large order folios come around would we a similar check
there?

So Willy, you would know better if and when a shared piece of code would
be best with all these things in mind.

  Luis