linux-kernel - Re: [PATCH v5 09/10] xfs: Allow block allocator to take an alignment hint

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Message-ID: <Z9GP6F_n2BR3XCn5@infradead.org>
Date: Wed, 12 Mar 2025 06:45:12 -0700
From: Christoph Hellwig <hch@...radead.org>
To: John Garry <john.g.garry@...cle.com>
Cc: Christoph Hellwig <hch@...radead.org>, brauner@...nel.org,
	djwong@...nel.org, cem@...nel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	ojaswin@...ux.ibm.com, ritesh.list@...il.com,
	martin.petersen@...cle.com
Subject: Re: [PATCH v5 09/10] xfs: Allow block allocator to take an alignment
 hint

On Wed, Mar 12, 2025 at 08:05:14AM +0000, John Garry wrote:
> > Shouldn't we be doing this by default for any extent size hint
> > based allocations?
> 
> I'm not sure.
> 
> I think that currently users just expect extszhint to hint at the
> granularity only.
> 
> Maybe users don't require alignment and adding an alignment requirement just
> leads to more fragmentation.

But does it?  Once an extsize hint is set I'd expect that we keep
getting more allocation with it.  And keeping the aligned is the concept
of a buddy allocator which reduces fragmentation.  Because of that I
wonder why we aren't doing that by default.