linux-kernel - Re: [PATCH v3 02/11] fs: Allow fine-grained control of folio sizes

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Date: Tue, 26 Mar 2024 09:44:02 +0100
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	gost.dev@...sung.com, chandan.babu@...cle.com, hare@...e.de, mcgrof@...nel.org, 
	djwong@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	david@...morbit.com, akpm@...ux-foundation.org, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v3 02/11] fs: Allow fine-grained control of folio sizes

On Mon, Mar 25, 2024 at 06:29:30PM +0000, Matthew Wilcox wrote:
> On Wed, Mar 13, 2024 at 06:02:44PM +0100, Pankaj Raghav (Samsung) wrote:
> > +/*
> > + * mapping_set_folio_min_order() - Set the minimum folio order
> > + * @mapping: The address_space.
> > + * @min: Minimum folio order (between 0-MAX_PAGECACHE_ORDER inclusive).
> > + *
> > + * The filesystem should call this function in its inode constructor to
> > + * indicate which base size of folio the VFS can use to cache the contents
> > + * of the file.  This should only be used if the filesystem needs special
> > + * handling of folio sizes (ie there is something the core cannot know).
> > + * Do not tune it based on, eg, i_size.
> > + *
> > + * Context: This should not be called while the inode is active as it
> > + * is non-atomic.
> > + */
> > +static inline void mapping_set_folio_min_order(struct address_space *mapping,
> > +					       unsigned int min)
> > +{
> > +	if (min > MAX_PAGECACHE_ORDER)
> > +		min = MAX_PAGECACHE_ORDER;
> > +
> > +	mapping->flags = (mapping->flags & ~AS_FOLIO_ORDER_MASK) |
> > +			 (min << AS_FOLIO_ORDER_MIN) |
> > +			 (MAX_PAGECACHE_ORDER << AS_FOLIO_ORDER_MAX);
> > +}
> 
> I was surprised when I read this, which indicates it might be surprising
> for others too.  I think it at least needs a comment to say that the
> maximum will be set to the MAX_PAGECACHE_ORDER, because I was expecting
> it to set max == min.  I guess that isn't what XFS wants, but someone
> doing this to, eg, ext4 is going to have an unpleasant surprise when
> they call into block_read_full_folio() and overrun 'arr'.
> 
> I'm still not entirely convinced this wouldn't be better to do as
> mapping_set_folio_order_range() and have
> 
> static inline void mapping_set_folio_min_order(struct address_space *mapping,
> 		unsigned int min)
> {
> 	mapping_set_folio_range(mapping, min, MAX_PAGECACHE_ORDER);
> }

I agree. Having a helper like this will make it more explicit. The
limits checking can also be done in this helper itself.

Also it makes mapping_set_large_folio() more clear:

static inline void mapping_set_large_folios(struct address_space *mapping)
{
      mapping_set_folio_range(mapping, 0, MAX_PAGECACHE_ORDER);
}

instead of just calling mapping_set_folio_min_order(). Thanks.