[Linux-cachefs] [PATCH v15 3/4] xfs: refactor the usage around xfs_trans_context_{set, clear}

[Yafang Shao]

The xfs_trans context should be active after it is allocated, and
deactive when it is freed.
The rolling transaction should be specially considered, because in the
case when we clear the old transaction the thread's NOFS state shouldn't
be changed, as a result we have to set NOFS in the old transaction's
t_pflags in xfs_trans_context_swap().

So these helpers are refactored as,
- xfs_trans_context_set()
  Used in xfs_trans_alloc()
- xfs_trans_context_clear()
  Used in xfs_trans_free()

And a new helper is instroduced to handle the rolling transaction,
- xfs_trans_context_swap()
  Used in rolling transaction

This patch is based on Darrick's work[1] to fix the issue in xfs/141 in the
earlier version and Dave's suggestion[2].

[1]. https://lore.kernel.org/linux-xfs/20201104001649.GN7123@magnolia
[2]. https://lore.kernel.org/linux-xfs/20201218000705.GR632069@dread.disaster.area

Cc: Dave Chinner <david at fromorbit.com>
Cc: Darrick J. Wong <darrick.wong at oracle.com>
Cc: Matthew Wilcox (Oracle) <willy at infradead.org>
Cc: Christoph Hellwig <hch at lst.de>
Signed-off-by: Yafang Shao <laoar.shao at gmail.com>
---
 fs/xfs/xfs_trans.c | 25 +++++++++++--------------
 fs/xfs/xfs_trans.h | 10 +++++++++-
 2 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
index 43107af569fe..b76e850c9ae7 100644
--- a/fs/xfs/xfs_trans.c
+++ b/fs/xfs/xfs_trans.c
@@ -67,6 +67,10 @@ xfs_trans_free(
 	xfs_extent_busy_sort(&tp->t_busy);
 	xfs_extent_busy_clear(tp->t_mountp, &tp->t_busy, false);
 
+
+	/* Detach the transaction from this thread. */
+	xfs_trans_context_clear(tp);
+
 	trace_xfs_trans_free(tp, _RET_IP_);
 	if (!(tp->t_flags & XFS_TRANS_NO_WRITECOUNT))
 		sb_end_intwrite(tp->t_mountp->m_super);
@@ -119,7 +123,9 @@ xfs_trans_dup(
 
 	ntp->t_rtx_res = tp->t_rtx_res - tp->t_rtx_res_used;
 	tp->t_rtx_res = tp->t_rtx_res_used;
-	ntp->t_pflags = tp->t_pflags;
+
+	/* Associate the new transaction with this thread. */
+	xfs_trans_context_swap(tp, ntp);
 
 	/* move deferred ops over to the new tp */
 	xfs_defer_move(ntp, tp);
@@ -153,9 +159,6 @@ xfs_trans_reserve(
 	int			error = 0;
 	bool			rsvd = (tp->t_flags & XFS_TRANS_RESERVE) != 0;
 
-	/* Mark this thread as being in a transaction */
-	xfs_trans_context_set(tp);
-
 	/*
 	 * Attempt to reserve the needed disk blocks by decrementing
 	 * the number needed from the number available.  This will
@@ -163,10 +166,9 @@ xfs_trans_reserve(
 	 */
 	if (blocks > 0) {
 		error = xfs_mod_fdblocks(mp, -((int64_t)blocks), rsvd);
-		if (error != 0) {
-			xfs_trans_context_clear(tp);
+		if (error != 0)
 			return -ENOSPC;
-		}
+
 		tp->t_blk_res += blocks;
 	}
 
@@ -241,8 +243,6 @@ xfs_trans_reserve(
 		tp->t_blk_res = 0;
 	}
 
-	xfs_trans_context_clear(tp);
-
 	return error;
 }
 
@@ -284,6 +284,8 @@ xfs_trans_alloc(
 	INIT_LIST_HEAD(&tp->t_dfops);
 	tp->t_firstblock = NULLFSBLOCK;
 
+	/* Mark this thread as being in a transaction */
+	xfs_trans_context_set(tp);
 	error = xfs_trans_reserve(tp, resp, blocks, rtextents);
 	if (error) {
 		xfs_trans_cancel(tp);
@@ -878,7 +880,6 @@ __xfs_trans_commit(
 
 	xfs_log_commit_cil(mp, tp, &commit_lsn, regrant);
 
-	xfs_trans_context_clear(tp);
 	xfs_trans_free(tp);
 
 	/*
@@ -911,7 +912,6 @@ __xfs_trans_commit(
 		tp->t_ticket = NULL;
 	}
 
-	xfs_trans_context_clear(tp);
 	xfs_trans_free_items(tp, !!error);
 	xfs_trans_free(tp);
 
@@ -971,9 +971,6 @@ xfs_trans_cancel(
 		tp->t_ticket = NULL;
 	}
 
-	/* mark this thread as no longer being in a transaction */
-	xfs_trans_context_clear(tp);
-
 	xfs_trans_free_items(tp, dirty);
 	xfs_trans_free(tp);
 }
diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
index 44b11c64a15e..3645fd0d74b8 100644
--- a/fs/xfs/xfs_trans.h
+++ b/fs/xfs/xfs_trans.h
@@ -277,7 +277,15 @@ xfs_trans_context_set(struct xfs_trans *tp)
 static inline void
 xfs_trans_context_clear(struct xfs_trans *tp)
 {
-	memalloc_nofs_restore(tp->t_pflags);
+	if (!tp->t_pflags)
+		memalloc_nofs_restore(tp->t_pflags);
+}
+
+static inline void
+xfs_trans_context_swap(struct xfs_trans *tp, struct xfs_trans *ntp)
+{
+	ntp->t_pflags = tp->t_pflags;
+	tp->t_pflags = -1;
 }
 
 #endif	/* __XFS_TRANS_H__ */
-- 
2.17.1