Received-SPF: pass (google.com: domain of linux-kernel+bounces-118565-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Tue, 26 Mar 2024 09:44:02 +0100
From: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, 
	gost.dev@samsung.com, chandan.babu@oracle.com, hare@suse.de, mcgrof@kernel.org, 
	djwong@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, 
	david@fromorbit.com, akpm@linux-foundation.org, Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [PATCH v3 02/11] fs: Allow fine-grained control of folio sizes
Message-ID: <hjkct6wz4amxzhr4ndrw7srnjepcr3kmd34kixynznhivxv5og@r7hdu4mqt3j3>
References: <20240313170253.2324812-1-kernel@pankajraghav.com>
 <20240313170253.2324812-3-kernel@pankajraghav.com>
 <ZgHCir0cpYZ4vOa0@casper.infradead.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZgHCir0cpYZ4vOa0@casper.infradead.org>

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.