Re: [PATCH v8 3/5] xfs: introduce xfs_ag_shrink_space()

On Mon, Mar 22, 2021 at 08:03:10PM +0800, Gao Xiang wrote:

Hi Brian,

On Mon, Mar 22, 2021 at 07:30:03AM -0400, Brian Foster wrote:

quoted

On Fri, Mar 05, 2021 at 10:57:01AM +0800, Gao Xiang wrote:

quoted

This patch introduces a helper to shrink unused space in the last AG
by fixing up the freespace btree.

Also make sure that the per-AG reservation works under the new AG
size. If such per-AG reservation or extent allocation fails, roll
the transaction so the new transaction could cancel without any side
effects.

Signed-off-by: Gao Xiang <redacted>
---

Looks mostly good to me. Some nits..

quoted

 fs/xfs/libxfs/xfs_ag.c | 111 +++++++++++++++++++++++++++++++++++++++++
 fs/xfs/libxfs/xfs_ag.h |   4 +-
 2 files changed, 114 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
index 9331f3516afa..1f6f9e70e1cb 100644
--- a/fs/xfs/libxfs/xfs_ag.c
+++ b/fs/xfs/libxfs/xfs_ag.c

...

quoted

@@ -485,6 +490,112 @@ xfs_ag_init_headers(
 	return error;
 }
 
+int
+xfs_ag_shrink_space(
+	struct xfs_mount	*mp,
+	struct xfs_trans	**tpp,
+	xfs_agnumber_t		agno,
+	xfs_extlen_t		delta)
+{
+	struct xfs_alloc_arg	args = {
+		.tp	= *tpp,
+		.mp	= mp,
+		.type	= XFS_ALLOCTYPE_THIS_BNO,
+		.minlen = delta,
+		.maxlen = delta,
+		.oinfo	= XFS_RMAP_OINFO_SKIP_UPDATE,
+		.resv	= XFS_AG_RESV_NONE,
+		.prod	= 1
+	};
+	struct xfs_buf		*agibp, *agfbp;
+	struct xfs_agi		*agi;
+	struct xfs_agf		*agf;
+	int			error, err2;
+
+	ASSERT(agno == mp->m_sb.sb_agcount - 1);
+	error = xfs_ialloc_read_agi(mp, *tpp, agno, &agibp);
+	if (error)
+		return error;
+
+	agi = agibp->b_addr;
+
+	error = xfs_alloc_read_agf(mp, *tpp, agno, 0, &agfbp);
+	if (error)
+		return error;
+
+	agf = agfbp->b_addr;
+	if (XFS_IS_CORRUPT(mp, agf->agf_length != agi->agi_length))
+		return -EFSCORRUPTED;

Is this check here for a reason? It seems a bit random, so I wonder if
we should just leave the extra verification to buffer verifiers.

It came from Darrick's thought. I'm fine with either way, but I feel
confused if different conflict opinions here:
https://lore.kernel.org/linux-xfs/20210303181931.GB3419940@magnolia/ (local)

Darrick's comment seems to refer to the check below. I'm referring to
the check above that agi_length and agf_length match. Are they intended
to go together? The check above seems to preexist the one below.

Anyways, if so, maybe just bunch them together and add a comment:

	/* some extra paranoid checks before we shrink the ag */
	if (XFS_IS_CORRUPT(...))
		return -EFSCORRUPTED;
	if (delta >= agf->agf_length)
		return -EVINAL; 

quoted

quoted

+
+	if (delta >= agi->agi_length)
+		return -EINVAL;
+
+	args.fsbno = XFS_AGB_TO_FSB(mp, agno,
+				    be32_to_cpu(agi->agi_length) - delta);
+
+	/* remove the preallocations before allocation and re-establish then */

...

quoted

quoted

diff --git a/fs/xfs/libxfs/xfs_ag.h b/fs/xfs/libxfs/xfs_ag.h
index 5166322807e7..41293ebde8da 100644
--- a/fs/xfs/libxfs/xfs_ag.h
+++ b/fs/xfs/libxfs/xfs_ag.h

@@ -24,8 +24,10 @@ struct aghdr_init_data {
 };
 
 int xfs_ag_init_headers(struct xfs_mount *mp, struct aghdr_init_data *id);
+int xfs_ag_shrink_space(struct xfs_mount *mp, struct xfs_trans **tpp,
+			xfs_agnumber_t agno, xfs_extlen_t len);
 int xfs_ag_extend_space(struct xfs_mount *mp, struct xfs_trans *tp,
-			struct aghdr_init_data *id, xfs_extlen_t len);
+			struct aghdr_init_data *id, xfs_extlen_t delta);

This looks misplaced..?

Or maybe this is trying to make the APIs consistent, but the function
definition still uses len as well as the declaration for
_ag_shrink_space() (while the definition of that function uses delta).

FWIW, the name delta tends to suggest a signed value to me based on our
pattern of usage, whereas here it seems like these helpers always want a
positive value (i.e. a length).

Yeah, it's just misplaced, thanks for pointing out, sorry about that.
`delta' name came from, `len' is confusing to Darrick.
https://lore.kernel.org/r/20210303182527.GC3419940@magnolia/ (local)

Fair enough. I'm not worried about the name, just pointing out some
potential inconsistencies.

Brian

Thanks,
Gao Xiang

quoted

Brian

quoted

 int xfs_ag_get_geometry(struct xfs_mount *mp, xfs_agnumber_t agno,
 			struct xfs_ag_geometry *ageo);
 
-- 
2.27.0