From: Dave Chinner <dchinner@redhat.com>

Currently the allocation at EOF is broken into two cases - when the
offset is zero and when the offset is non-zero. When the offset is
non-zero, we try to do exact block allocation for contiguous
extent allocation. When the offset is zero, the allocation is simply
an aligned allocation.

We want aligned allocation as the fallback when exact block
allocation fails, but that complicates the EOF allocation in that it
now has to handle two different allocation cases. The
caller also has to handle allocation when not at EOF, and for the
upcoming forced alignment changes we need that to also be aligned
allocation.

To simplify all this, pull the aligned allocation cases back into
the callers and leave the EOF allocation path for exact block
allocation only. This means that the EOF exact block allocation
fallback path is the normal aligned allocation path and that ends up
making things a lot simpler when forced alignment is introduced.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 fs/xfs/libxfs/xfs_bmap.c   | 129 +++++++++++++++----------------------
 fs/xfs/libxfs/xfs_ialloc.c |   2 +-
 2 files changed, 54 insertions(+), 77 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index 25a87e1154bb..42a75d257b35 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -3310,12 +3310,12 @@ xfs_bmap_select_minlen(
 static int
 xfs_bmap_btalloc_select_lengths(
 	struct xfs_bmalloca	*ap,
-	struct xfs_alloc_arg	*args,
-	xfs_extlen_t		*blen)
+	struct xfs_alloc_arg	*args)
 {
 	struct xfs_mount	*mp = args->mp;
 	struct xfs_perag	*pag;
 	xfs_agnumber_t		agno, startag;
+	xfs_extlen_t		blen = 0;
 	int			error = 0;
 
 	if (ap->tp->t_flags & XFS_TRANS_LOWMODE) {
@@ -3329,19 +3329,18 @@ xfs_bmap_btalloc_select_lengths(
 	if (startag == NULLAGNUMBER)
 		startag = 0;
 
-	*blen = 0;
 	for_each_perag_wrap(mp, startag, agno, pag) {
-		error = xfs_bmap_longest_free_extent(pag, args->tp, blen);
+		error = xfs_bmap_longest_free_extent(pag, args->tp, &blen);
 		if (error && error != -EAGAIN)
 			break;
 		error = 0;
-		if (*blen >= args->maxlen)
+		if (blen >= args->maxlen)
 			break;
 	}
 	if (pag)
 		xfs_perag_rele(pag);
 
-	args->minlen = xfs_bmap_select_minlen(ap, args, *blen);
+	args->minlen = xfs_bmap_select_minlen(ap, args, blen);
 	return error;
 }
 
@@ -3551,78 +3550,40 @@ xfs_bmap_exact_minlen_extent_alloc(
  * If we are not low on available data blocks and we are allocating at
  * EOF, optimise allocation for contiguous file extension and/or stripe
  * alignment of the new extent.
- *
- * NOTE: ap->aeof is only set if the allocation length is >= the
- * stripe unit and the allocation offset is at the end of file.
  */
 static int
 xfs_bmap_btalloc_at_eof(
 	struct xfs_bmalloca	*ap,
-	struct xfs_alloc_arg	*args,
-	xfs_extlen_t		blen,
-	bool			ag_only)
+	struct xfs_alloc_arg	*args)
 {
 	struct xfs_mount	*mp = args->mp;
 	struct xfs_perag	*caller_pag = args->pag;
+	xfs_extlen_t		alignment = args->alignment;
 	int			error;
 
+	ASSERT(ap->aeof && ap->offset);
+	ASSERT(args->alignment >= 1);
+
 	/*
-	 * If there are already extents in the file, try an exact EOF block
-	 * allocation to extend the file as a contiguous extent. If that fails,
-	 * or it's the first allocation in a file, just try for a stripe aligned
-	 * allocation.
+	 * Compute the alignment slop for the fallback path so we ensure
+	 * we account for the potential alignemnt space required by the
+	 * fallback paths before we modify the AGF and AGFL here.
 	 */
-	if (ap->offset) {
-		xfs_extlen_t	alignment = args->alignment;
-
-		/*
-		 * Compute the alignment slop for the fallback path so we ensure
-		 * we account for the potential alignment space required by the
-		 * fallback paths before we modify the AGF and AGFL here.
-		 */
-		args->alignment = 1;
-		args->alignslop = alignment - args->alignment;
-
-		if (!caller_pag)
-			args->pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, ap->blkno));
-		error = xfs_alloc_vextent_exact_bno(args, ap->blkno);
-		if (!caller_pag) {
-			xfs_perag_put(args->pag);
-			args->pag = NULL;
-		}
-		if (error)
-			return error;
-
-		if (args->fsbno != NULLFSBLOCK)
-			return 0;
-		/*
-		 * Exact allocation failed. Reset to try an aligned allocation
-		 * according to the original allocation specification.
-		 */
-		args->alignment = alignment;
-		args->alignslop = 0;
-	}
+	args->alignment = 1;
+	args->alignslop = alignment - args->alignment;
 
-	if (ag_only) {
-		error = xfs_alloc_vextent_near_bno(args, ap->blkno);
-	} else {
+	if (!caller_pag)
+		args->pag = xfs_perag_get(mp, XFS_FSB_TO_AGNO(mp, ap->blkno));
+	error = xfs_alloc_vextent_exact_bno(args, ap->blkno);
+	if (!caller_pag) {
+		xfs_perag_put(args->pag);
 		args->pag = NULL;
-		error = xfs_alloc_vextent_start_ag(args, ap->blkno);
-		ASSERT(args->pag == NULL);
-		args->pag = caller_pag;
 	}
-	if (error)
-		return error;
 
-	if (args->fsbno != NULLFSBLOCK)
-		return 0;
-
-	/*
-	 * Aligned allocation failed, so all fallback paths from here drop the
-	 * start alignment requirement as we know it will not succeed.
-	 */
-	args->alignment = 1;
-	return 0;
+	/* Reset alignment to original specifications.  */
+	args->alignment = alignment;
+	args->alignslop = 0;
+	return error;
 }
 
 /*
@@ -3688,12 +3649,19 @@ xfs_bmap_btalloc_filestreams(
 	}
 
 	args->minlen = xfs_bmap_select_minlen(ap, args, blen);
-	if (ap->aeof)
-		error = xfs_bmap_btalloc_at_eof(ap, args, blen, true);
+	if (ap->aeof && ap->offset)
+		error = xfs_bmap_btalloc_at_eof(ap, args);
 
+	/* This may be an aligned allocation attempt. */
 	if (!error && args->fsbno == NULLFSBLOCK)
 		error = xfs_alloc_vextent_near_bno(args, ap->blkno);
 
+	/* Attempt non-aligned allocation if we haven't already. */
+	if (!error && args->fsbno == NULLFSBLOCK && args->alignment > 1)  {
+		args->alignment = 1;
+		error = xfs_alloc_vextent_near_bno(args, ap->blkno);
+	}
+
 out_low_space:
 	/*
 	 * We are now done with the perag reference for the filestreams
@@ -3715,7 +3683,6 @@ xfs_bmap_btalloc_best_length(
 	struct xfs_bmalloca	*ap,
 	struct xfs_alloc_arg	*args)
 {
-	xfs_extlen_t		blen = 0;
 	int			error;
 
 	ap->blkno = XFS_INO_TO_FSB(args->mp, ap->ip->i_ino);
@@ -3726,23 +3693,33 @@ xfs_bmap_btalloc_best_length(
 	 * the request.  If one isn't found, then adjust the minimum allocation
 	 * size to the largest space found.
 	 */
-	error = xfs_bmap_btalloc_select_lengths(ap, args, &blen);
+	error = xfs_bmap_btalloc_select_lengths(ap, args);
 	if (error)
 		return error;
 
 	/*
-	 * Don't attempt optimal EOF allocation if previous allocations barely
-	 * succeeded due to being near ENOSPC. It is highly unlikely we'll get
-	 * optimal or even aligned allocations in this case, so don't waste time
-	 * trying.
+	 * If we are in low space mode, then optimal allocation will fail so
+	 * prepare for minimal allocation and run the low space algorithm
+	 * immediately.
 	 */
-	if (ap->aeof && !(ap->tp->t_flags & XFS_TRANS_LOWMODE)) {
-		error = xfs_bmap_btalloc_at_eof(ap, args, blen, false);
-		if (error || args->fsbno != NULLFSBLOCK)
-			return error;
+	if (ap->tp->t_flags & XFS_TRANS_LOWMODE) {
+		ASSERT(args->fsbno == NULLFSBLOCK);
+		return xfs_bmap_btalloc_low_space(ap, args);
+	}
+
+	if (ap->aeof && ap->offset)
+		error = xfs_bmap_btalloc_at_eof(ap, args);
+
+	/* This may be an aligned allocation attempt. */
+	if (!error && args->fsbno == NULLFSBLOCK)
+		error = xfs_alloc_vextent_start_ag(args, ap->blkno);
+
+	/* Attempt non-aligned allocation if we haven't already. */
+	if (!error && args->fsbno == NULLFSBLOCK && args->alignment > 1)  {
+		args->alignment = 1;
+		error = xfs_alloc_vextent_start_ag(args, ap->blkno);
 	}
 
-	error = xfs_alloc_vextent_start_ag(args, ap->blkno);
 	if (error || args->fsbno != NULLFSBLOCK)
 		return error;
 
diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
index 2fa29d2f004e..c5d220d51757 100644
--- a/fs/xfs/libxfs/xfs_ialloc.c
+++ b/fs/xfs/libxfs/xfs_ialloc.c
@@ -780,7 +780,7 @@ xfs_ialloc_ag_alloc(
 		 * the exact agbno requirement and increase the alignment
 		 * instead. It is critical that the total size of the request
 		 * (len + alignment + slop) does not increase from this point
-		 * on, so reset minalignslop to ensure it is not included in
+		 * on, so reset alignslop to ensure it is not included in
 		 * subsequent requests.
 		 */
 		args.alignslop = 0;
-- 
2.31.1

John Garry <john.g.garry@oracle.com> writes:

> From: Dave Chinner <dchinner@redhat.com>
>
> Currently the allocation at EOF is broken into two cases - when the
> offset is zero and when the offset is non-zero. When the offset is
> non-zero, we try to do exact block allocation for contiguous
> extent allocation. When the offset is zero, the allocation is simply
> an aligned allocation.
>
> We want aligned allocation as the fallback when exact block
> allocation fails, but that complicates the EOF allocation in that it
> now has to handle two different allocation cases. The
> caller also has to handle allocation when not at EOF, and for the
> upcoming forced alignment changes we need that to also be aligned
> allocation.
>
> To simplify all this, pull the aligned allocation cases back into
> the callers and leave the EOF allocation path for exact block
> allocation only. This means that the EOF exact block allocation
> fallback path is the normal aligned allocation path and that ends up
> making things a lot simpler when forced alignment is introduced.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  fs/xfs/libxfs/xfs_bmap.c   | 129 +++++++++++++++----------------------
>  fs/xfs/libxfs/xfs_ialloc.c |   2 +-
>  2 files changed, 54 insertions(+), 77 deletions(-)
>
<..>

> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
> index 2fa29d2f004e..c5d220d51757 100644
> --- a/fs/xfs/libxfs/xfs_ialloc.c
> +++ b/fs/xfs/libxfs/xfs_ialloc.c
> @@ -780,7 +780,7 @@ xfs_ialloc_ag_alloc(
>  		 * the exact agbno requirement and increase the alignment
>  		 * instead. It is critical that the total size of the request
>  		 * (len + alignment + slop) does not increase from this point
> -		 * on, so reset minalignslop to ensure it is not included in
> +		 * on, so reset alignslop to ensure it is not included in
>  		 * subsequent requests.
>  		 */
>  		args.alignslop = 0;

minor comment: Looks like this diff got leftover from previous patch
where we cleanup minalignslop/alignslop.

-ritesh

On 04/09/2024 19:25, Ritesh Harjani (IBM) wrote:
>> diff --git a/fs/xfs/libxfs/xfs_ialloc.c b/fs/xfs/libxfs/xfs_ialloc.c
>> index 2fa29d2f004e..c5d220d51757 100644
>> --- a/fs/xfs/libxfs/xfs_ialloc.c
>> +++ b/fs/xfs/libxfs/xfs_ialloc.c
>> @@ -780,7 +780,7 @@ xfs_ialloc_ag_alloc(
>>   		 * the exact agbno requirement and increase the alignment
>>   		 * instead. It is critical that the total size of the request
>>   		 * (len + alignment + slop) does not increase from this point
>> -		 * on, so reset minalignslop to ensure it is not included in
>> +		 * on, so reset alignslop to ensure it is not included in
>>   		 * subsequent requests.
>>   		 */
>>   		args.alignslop = 0;
> minor comment: Looks like this diff got leftover from previous patch
> where we cleanup minalignslop/alignslop.

Right, that comment modification belongs in the previous patch - I will 
relocate.

Thanks,
John