Re: [PATCH v3 6/8] filemap: Allow __filemap_get_folio to allocate large folios

[Dave Chinner <david@fromorbit.com>]

On Fri, Jun 16, 2023 at 06:45:43PM +0100, Matthew Wilcox wrote:
> On Tue, Jun 13, 2023 at 05:54:35PM +1000, Dave Chinner wrote:
> > If I hadn't looked at the code closely and saw a trace with this
> > sort of behaviour (i.e. I understood large folios were in use,
> > but not exactly how they worked), I'd be very surprised to see a
> > weird repeated pattern of varying folio sizes. I'd probably think
> > it was a bug in the implementation....
> > 
> > > I'd prefer the low-risk approach for now; we can change it later!
> > 
> > That's fine by me - just document the limitations and expected
> > behaviour in the code rather than expect people to have to discover
> > this behaviour for themselves.
> 
> How about this?
> 
> +++ b/include/linux/pagemap.h
> @@ -548,6 +548,17 @@ typedef unsigned int __bitwise fgf_t;
> 
>  #define FGP_WRITEBEGIN         (FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
> 
> +/**
> + * fgf_set_order - Encode a length in the fgf_t flags.
> + * @size: The suggested size of the folio to create.
> + *
> + * The caller of __filemap_get_folio() can use this to suggest a preferred
> + * size for the folio that is created.  If there is already a folio at
> + * the index, it will be returned, no matter what its size.  If a folio
> + * is freshly created, it may be of a different size than requested
> + * due to alignment constraints, memory pressure, or the presence of
> + * other folios at nearby indices.
> + */
>  static inline fgf_t fgf_set_order(size_t size)
>  {
>         unsigned int shift = ilog2(size);

Yup, I'm happy with that - "preferred size" is a good way of
describing the best effort attempt it makes....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com