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

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Date: Thu, 15 Feb 2024 11:34:24 +0100
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: "Darrick J. Wong" <djwong@...nel.org>, 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, linux-mm@...ck.org, david@...morbit.com
Subject: Re: [RFC v2 01/14] fs: Allow fine-grained control of folio sizes

> > > 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.
> 
> I favour introducing the right APIs even if they're not fully implemented.
> We have no filesystems today that need this, so it doesn't need to
> be implemented, but if we have to go back and add it, it's more churn
> for every filesystem.  I'm open to better ideas about the API; I think
> for a lot of filesystems they only want to set the minimum, so maybe
> introducing that API now would be a good thing.

I will introduce a new API that only exposes the min order for now. I
agree with you that I don't see a lot of filesystems other than XFS
using this in the near future.

We deduce min order based on the filesystem blocksize but we don't have any
mechanisms in place from userspace to set the max order for a filesystem.
So that also needs to be thought through and discussed with the
community.

I hope to start working on max_order immediately after upstreaming the
min_order feature.

--
Pankaj