linux-kernel - Re: [PATCH v2 02/14] fs: xfs: Don't use low-space allocator for alignment

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Message-ID: <ZeZIB0G3zjaq7dWK@dread.disaster.area>
Date: Tue, 5 Mar 2024 09:15:35 +1100
From: Dave Chinner <david@...morbit.com>
To: John Garry <john.g.garry@...cle.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk,
	brauner@...nel.org, jack@...e.cz, chandan.babu@...cle.com,
	axboe@...nel.dk, martin.petersen@...cle.com,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
	ojaswin@...ux.ibm.com, ritesh.list@...il.com,
	linux-block@...r.kernel.org
Subject: Re: [PATCH v2 02/14] fs: xfs: Don't use low-space allocator for
 alignment > 1

On Mon, Mar 04, 2024 at 01:04:16PM +0000, John Garry wrote:
> The low-space allocator doesn't honour the alignment requirement, so don't
> attempt to even use it (when we have an alignment requirement).
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
>  fs/xfs/libxfs/xfs_bmap.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index f362345467fa..60d100134280 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -3584,6 +3584,10 @@ xfs_bmap_btalloc_low_space(
>  {
>  	int			error;
>  
> +	/* The allocator doesn't honour args->alignment */
> +	if (args->alignment > 1)
> +		return 0;

I think that's wrong.

The alignment argument here is purely a best effort consideration -
we ignore it several different allocation situations, not just low
space.

e.g. xfs_bmap_btalloc_at_eof() will try exact block
allocation regardless of whether an alignment parameter is set. It
will then fall back to stripe alignment if exact block fails.

If stripe aligned allocation fails, it will then set args->alignment
= 1 and try a full filesystem allocation scan without alignment.

And if that fails, then we finally get to the low space allocator
with args->alignment = 1 even though we might be trying to allocate
an aligned extent for an atomic IO....

IOWs, I think this indicates deeper surgery is needed to ensure
aligned allocations fail immediately and don't fall back to
unaligned allocations and set XFS_TRANS_LOW_MODE...

-Dave.
-- 
Dave Chinner
david@...morbit.com