linux-kernel - Re: [PATCH 01/13] xfs: only allow minlen allocations when near ENOSPC

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Date: Fri, 21 Jun 2024 12:42:25 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: chandan.babu@...cle.com, dchinner@...hat.com, hch@....de,
	viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, catherine.hoang@...cle.com,
	martin.petersen@...cle.com
Subject: Re: [PATCH 01/13] xfs: only allow minlen allocations when near ENOSPC

On Fri, Jun 21, 2024 at 10:05:28AM +0000, John Garry wrote:
> From: Dave Chinner <dchinner@...hat.com>
> 
> When we are near ENOSPC and don't have enough free
> space for an args->maxlen allocation, xfs_alloc_space_available()
> will trim args->maxlen to equal the available space. However, this
> function has only checked that there is enough contiguous free space
> for an aligned args->minlen allocation to succeed. Hence there is no
> guarantee that an args->maxlen allocation will succeed, nor that the
> available space will allow for correct alignment of an args->maxlen
> allocation.
> 
> Further, by trimming args->maxlen arbitrarily, it breaks an
> assumption made in xfs_alloc_fix_len() that if the caller wants
> aligned allocation, then args->maxlen will be set to an aligned
> value. It then skips the tail alignment and so we end up with
> extents that aren't aligned to extent size hint boundaries as we
> approach ENOSPC.
> 
> To avoid this problem, don't reduce args->maxlen by some random,
> arbitrary amount. If args->maxlen is too large for the available
> space, reduce the allocation to a minlen allocation as we know we
> have contiguous free space available for this to succeed and always
> be correctly aligned.
> 
> Signed-off-by: Dave Chinner <dchinner@...hat.com>
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
>  fs/xfs/libxfs/xfs_alloc.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
> index 6c55a6e88eba..5855a21d4864 100644
> --- a/fs/xfs/libxfs/xfs_alloc.c
> +++ b/fs/xfs/libxfs/xfs_alloc.c
> @@ -2409,14 +2409,23 @@ xfs_alloc_space_available(
>  	if (available < (int)max(args->total, alloc_len))
>  		return false;
>  
> +	if (flags & XFS_ALLOC_FLAG_CHECK)
> +		return true;
> +
>  	/*
> -	 * Clamp maxlen to the amount of free space available for the actual
> -	 * extent allocation.
> +	 * If we can't do a maxlen allocation, then we must reduce the size of
> +	 * the allocation to match the available free space. We know how big
> +	 * the largest contiguous free space we can allocate is, so that's our
> +	 * upper bound. However, we don't exaclty know what alignment/size
> +	 * constraints have been placed on the allocation, so we can't
> +	 * arbitrarily select some new max size. Hence make this a minlen
> +	 * allocation as we know that will definitely succeed and match the
> +	 * callers alignment constraints.
>  	 */
> -	if (available < (int)args->maxlen && !(flags & XFS_ALLOC_FLAG_CHECK)) {
> -		args->maxlen = available;
> +	alloc_len = args->maxlen + (args->alignment - 1) + args->minalignslop;

Didn't we already calculate alloc_len identically under "do we have
enough contiguous free space for the allocation?"?  AFAICT we haven't
alter anything in @args since then, right?

> +	if (longest < alloc_len) {
> +		args->maxlen = args->minlen;

Is it possible to reduce maxlen the largest multiple of the alignment
that is still less than @longest?

--D

>  		ASSERT(args->maxlen > 0);
> -		ASSERT(args->maxlen >= args->minlen);
>  	}
>  
>  	return true;
> -- 
> 2.31.1
> 
>