[PATCH 3/6] xfs: commit CoW-based atomic writes atomically

["Darrick J. Wong" <djwong@xxxxxxxxxx>]

From: John Garry <john.g.garry@xxxxxxxxxx>

Source kernel commit: b1e09178b73adf10dc87fba9aee7787a7ad26874

When completing a CoW-based write, each extent range mapping update is
covered by a separate transaction.

For a CoW-based atomic write, all mappings must be changed at once, so
change to use a single transaction.

Note that there is a limit on the amount of log intent items which can be
fit into a single transaction, but this is being ignored for now since
the count of items for a typical atomic write would be much less than is
typically supported. A typical atomic write would be expected to be 64KB
or less, which means only 16 possible extents unmaps, which is quite
small.

Reviewed-by: Darrick J. Wong <djwong@xxxxxxxxxx>
[djwong: add tr_atomic_ioend]
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx>
Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
 libxfs/xfs_trans_resv.h |    1 +
 libxfs/xfs_log_rlimit.c |    4 ++++
 libxfs/xfs_trans_resv.c |   15 +++++++++++++++
 3 files changed, 20 insertions(+)


diff --git a/libxfs/xfs_trans_resv.h b/libxfs/xfs_trans_resv.h
index d9d0032cbbc5d4..670045d417a65f 100644
--- a/libxfs/xfs_trans_resv.h
+++ b/libxfs/xfs_trans_resv.h
@@ -48,6 +48,7 @@ struct xfs_trans_resv {
 	struct xfs_trans_res	tr_qm_dqalloc;	/* allocate quota on disk */
 	struct xfs_trans_res	tr_sb;		/* modify superblock */
 	struct xfs_trans_res	tr_fsyncts;	/* update timestamps on fsync */
+	struct xfs_trans_res	tr_atomic_ioend; /* untorn write completion */
 };
 
 /* shorthand way of accessing reservation structure */
diff --git a/libxfs/xfs_log_rlimit.c b/libxfs/xfs_log_rlimit.c
index 246d5f486d024a..2b9047f5ffb58b 100644
--- a/libxfs/xfs_log_rlimit.c
+++ b/libxfs/xfs_log_rlimit.c
@@ -91,6 +91,7 @@ xfs_log_calc_trans_resv_for_minlogblocks(
 	 */
 	if (xfs_want_minlogsize_fixes(&mp->m_sb)) {
 		xfs_trans_resv_calc(mp, resv);
+		resv->tr_atomic_ioend = M_RES(mp)->tr_atomic_ioend;
 		return;
 	}
 
@@ -107,6 +108,9 @@ xfs_log_calc_trans_resv_for_minlogblocks(
 
 	xfs_trans_resv_calc(mp, resv);
 
+	/* Copy the dynamic transaction reservation types from the running fs */
+	resv->tr_atomic_ioend = M_RES(mp)->tr_atomic_ioend;
+
 	if (xfs_has_reflink(mp)) {
 		/*
 		 * In the early days of reflink, typical log operation counts
diff --git a/libxfs/xfs_trans_resv.c b/libxfs/xfs_trans_resv.c
index 0a843deb50a118..cf735f946c8ac7 100644
--- a/libxfs/xfs_trans_resv.c
+++ b/libxfs/xfs_trans_resv.c
@@ -1281,6 +1281,15 @@ xfs_calc_namespace_reservations(
 	resp->tr_mkdir.tr_logflags |= XFS_TRANS_PERM_LOG_RES;
 }
 
+STATIC void
+xfs_calc_default_atomic_ioend_reservation(
+	struct xfs_mount	*mp,
+	struct xfs_trans_resv	*resp)
+{
+	/* Pick a default that will scale reasonably for the log size. */
+	resp->tr_atomic_ioend = resp->tr_itruncate;
+}
+
 void
 xfs_trans_resv_calc(
 	struct xfs_mount	*mp,
@@ -1375,4 +1384,10 @@ xfs_trans_resv_calc(
 	resp->tr_itruncate.tr_logcount += logcount_adj;
 	resp->tr_write.tr_logcount += logcount_adj;
 	resp->tr_qm_dqalloc.tr_logcount += logcount_adj;
+
+	/*
+	 * Now that we've finished computing the static reservations, we can
+	 * compute the dynamic reservation for atomic writes.
+	 */
+	xfs_calc_default_atomic_ioend_reservation(mp, resp);
 }