Re: [RFC v2 01/14] fs: Allow fine-grained control of folio sizes

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Feb 13, 2024 at 10:05:54PM +0100, Pankaj Raghav (Samsung) wrote:
> On Tue, Feb 13, 2024 at 08:34:31AM -0800, Darrick J. Wong wrote:
> > On Tue, Feb 13, 2024 at 10:37:00AM +0100, Pankaj Raghav (Samsung) wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > > 
> > > Some filesystems want to be able to limit the maximum size of folios,
> > > and some want to be able to ensure that folios are at least a certain
> > > size.  Add mapping_set_folio_orders() to allow this level of control.
> > > The max folio order parameter is ignored and it is always set to
> > > MAX_PAGECACHE_ORDER.
> > 
> > Why?  If MAX_PAGECACHE_ORDER is 8 and I instead pass in max==3, I'm
> > going to be surprised by my constraint being ignored.  Maybe I said that
> > because I'm not prepared to handle an order-7 folio; or some customer
> > will have some weird desire to twist this knob to make their workflow
> > faster.
> > 
> > --D
> Maybe I should have been explicit. We are planning to add support
> for min order in the first round, and we want to add support for max order
> once the min order support is upstreamed. It was done mainly to reduce
> the scope and testing of this series.
> 
> I definitely agree there are usecases for setting the max order. It is
> also the feedback we got from LPC.
> 
> So one idea would be not to expose max option until we add the support
> for max order? So filesystems can only set the min_order with the
> initial support?

Yeah, there's really no point in having an argument that's deliberately
ignored.

--D

> > > +static inline void mapping_set_folio_orders(struct address_space *mapping,
> > > +					    unsigned int min, unsigned int max)
> > > +{
> > > +	if (min == 1)
> > > +		min = 2;
> > > +	if (max < min)
> > > +		max = min;
> > > +	if (max > MAX_PAGECACHE_ORDER)
> > > +		max = MAX_PAGECACHE_ORDER;
> > > +
> > > +	/*
> > > +	 * XXX: max is ignored as only minimum folio order is supported
> > > +	 * currently.
> > > +	 */
> > > +	mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
> > > +			 (min << AS_FOLIO_ORDER_MIN) |
> > > +			 (MAX_PAGECACHE_ORDER << AS_FOLIO_ORDER_MAX);
> > > +}
> > > +
>