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Message-ID: <20240213212914.GW616564@frogsfrogsfrogs>
Date: Tue, 13 Feb 2024 13:29:14 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
mcgrof@...nel.org, gost.dev@...sung.com, akpm@...ux-foundation.org,
kbusch@...nel.org, chandan.babu@...cle.com, p.raghav@...sung.com,
linux-kernel@...r.kernel.org, hare@...e.de, willy@...radead.org,
linux-mm@...ck.org, david@...morbit.com
Subject: Re: [RFC v2 01/14] fs: Allow fine-grained control of folio sizes
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@...radead.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);
> > > +}
> > > +
>
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