linux-kernel - Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

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Message-ID: <Zoa9rQbEUam467-q@casper.infradead.org>
Date: Thu, 4 Jul 2024 16:20:13 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>,
	david@...morbit.com, chandan.babu@...cle.com, djwong@...nel.org,
	brauner@...nel.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, yang@...amperecomputing.com,
	linux-mm@...ck.org, john.g.garry@...cle.com,
	linux-fsdevel@...r.kernel.org, hare@...e.de, p.raghav@...sung.com,
	mcgrof@...nel.org, gost.dev@...sung.com, cl@...amperecomputing.com,
	linux-xfs@...r.kernel.org, hch@....de, Zi Yan <zi.yan@...t.com>
Subject: Re: [PATCH v8 01/10] fs: Allow fine-grained control of folio sizes

On Thu, Jul 04, 2024 at 01:23:20PM +0100, Ryan Roberts wrote:
> > -	AS_LARGE_FOLIO_SUPPORT = 6,
> 
> nit: this removed enum is still referenced in a comment further down the file.

Thanks.  Pankaj, let me know if you want me to send you a patch or if
you'll do it directly.

> > +	/* Bits 16-25 are used for FOLIO_ORDER */
> > +	AS_FOLIO_ORDER_BITS = 5,
> > +	AS_FOLIO_ORDER_MIN = 16,
> > +	AS_FOLIO_ORDER_MAX = AS_FOLIO_ORDER_MIN + AS_FOLIO_ORDER_BITS,
> 
> nit: These 3 new enums seem a bit odd.

Yes, this is "too many helpful suggestions" syndrome.  It made a lot
more sense originally.

https://lore.kernel.org/linux-fsdevel/ZlUQcEaP3FDXpCge@dread.disaster.area/

> > +static inline void mapping_set_folio_order_range(struct address_space *mapping,
> > +						 unsigned int min,
> > +						 unsigned int max)
> > +{
> > +	if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE))
> > +		return;
> > +
> > +	if (min > MAX_PAGECACHE_ORDER)
> > +		min = MAX_PAGECACHE_ORDER;
> > +	if (max > MAX_PAGECACHE_ORDER)
> > +		max = MAX_PAGECACHE_ORDER;
> > +	if (max < min)
> > +		max = min;
> 
> It seems strange to silently clamp these? Presumably for the bs>ps usecase,
> whatever values are passed in are a hard requirement? So wouldn't want them to
> be silently reduced. (Especially given the recent change to reduce the size of
> MAX_PAGECACHE_ORDER to less then PMD size in some cases).

Hm, yes.  We should probably make this return an errno.  Including
returning an errno for !IS_ENABLED() and min > 0.

> > -	if (new_order < MAX_PAGECACHE_ORDER) {
> > +	if (new_order < mapping_max_folio_order(mapping)) {
> >  		new_order += 2;
> > -		new_order = min_t(unsigned int, MAX_PAGECACHE_ORDER, new_order);
> > +		new_order = min(mapping_max_folio_order(mapping), new_order);
> >  		new_order = min_t(unsigned int, new_order, ilog2(ra->size));
> 
> I wonder if its possible that ra->size could ever be less than
> mapping_min_folio_order()? Do you need to handle that?

I think we take care of that in later patches?  This patch is mostly
about honouring the max properly and putting in the infrastructure for
the min, but not doing all the necessary work for min.