[Linux-cachefs] [PATCH v13 4/4] xfs: use current->journal_info to avoid transaction reservation recursion

[Yafang Shao]

PF_FSTRANS which is used to avoid transaction reservation recursion, is
dropped since commit 9070733b4efa ("xfs: abstract PF_FSTRANS to
PF_MEMALLOC_NOFS") and replaced by PF_MEMALLOC_NOFS which means to avoid
filesystem reclaim recursion.

As these two flags have different meanings, we'd better reintroduce
PF_FSTRANS back. To avoid wasting the space of PF_* flags in task_struct,
we can reuse the current->journal_info to do that, per Willy. As the
check of transaction reservation recursion is used by XFS only, we can
move the check into xfs_vm_writepage(s), per Dave.

Cc: Darrick J. Wong <darrick.wong at oracle.com>
Cc: Matthew Wilcox (Oracle) <willy at infradead.org>
Cc: Christoph Hellwig <hch at lst.de>
Cc: Dave Chinner <david at fromorbit.com>
Cc: Michal Hocko <mhocko at kernel.org>
Cc: David Howells <dhowells at redhat.com>
Cc: Jeff Layton <jlayton at redhat.com>
Signed-off-by: Yafang Shao <laoar.shao at gmail.com>
---
 fs/iomap/buffered-io.c |  7 -------
 fs/xfs/xfs_aops.c      | 17 +++++++++++++++++
 fs/xfs/xfs_trans.h     | 26 +++++++++++++++++++-------
 3 files changed, 36 insertions(+), 14 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 10cc7979ce38..3c53fa6ce64d 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1458,13 +1458,6 @@ iomap_do_writepage(struct page *page, struct writeback_control *wbc, void *data)
 			PF_MEMALLOC))
 		goto redirty;
 
-	/*
-	 * Given that we do not allow direct reclaim to call us, we should
-	 * never be called in a recursive filesystem reclaim context.
-	 */
-	if (WARN_ON_ONCE(current->flags & PF_MEMALLOC_NOFS))
-		goto redirty;
-
 	/*
 	 * Is this page beyond the end of the file?
 	 *
diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 2371187b7615..0da0242d42c3 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -568,6 +568,16 @@ xfs_vm_writepage(
 {
 	struct xfs_writepage_ctx wpc = { };
 
+	/*
+	 * Given that we do not allow direct reclaim to call us, we should
+	 * never be called while in a filesystem transaction.
+	 */
+	if (WARN_ON_ONCE(xfs_trans_context_active())) {
+		redirty_page_for_writepage(wbc, page);
+		unlock_page(page);
+		return 0;
+	}
+
 	return iomap_writepage(page, wbc, &wpc.ctx, &xfs_writeback_ops);
 }
 
@@ -579,6 +589,13 @@ xfs_vm_writepages(
 	struct xfs_writepage_ctx wpc = { };
 
 	xfs_iflags_clear(XFS_I(mapping->host), XFS_ITRUNCATED);
+	/*
+	 * Given that we do not allow direct reclaim to call us, we should
+	 * never be called while in a filesystem transaction.
+	 */
+	if (WARN_ON_ONCE(xfs_trans_context_active()))
+		return 0;
+
 	return iomap_writepages(mapping, wbc, &wpc.ctx, &xfs_writeback_ops);
 }
 
diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
index 12380eaaf7ce..0c8140147b9b 100644
--- a/fs/xfs/xfs_trans.h
+++ b/fs/xfs/xfs_trans.h
@@ -268,29 +268,41 @@ xfs_trans_item_relog(
 	return lip->li_ops->iop_relog(lip, tp);
 }
 
+static inline bool
+xfs_trans_context_active(void)
+{
+	/* Use journal_info to indicate current is in a transaction */
+	return current->journal_info != NULL;
+}
+
 static inline void
 xfs_trans_context_set(struct xfs_trans *tp)
 {
+	ASSERT(!current->journal_info);
+	current->journal_info = tp;
 	tp->t_pflags = memalloc_nofs_save();
 }
 
 static inline void
 xfs_trans_context_clear(struct xfs_trans *tp)
 {
+	/*
+	 * If xfs_trans_context_swap() handed the NOFS context to a
+	 * new transaction we do not clear the context here.
+	 */
+	if (current->journal_info != tp)
+		return;
+
+	current->journal_info = NULL;
 	memalloc_nofs_restore(tp->t_pflags);
 }
 
 static inline void
 xfs_trans_context_swap(struct xfs_trans *tp, struct xfs_trans *ntp)
 {
+	ASSERT(current->journal_info == tp);
+	current->journal_info = ntp;
 	ntp->t_pflags = tp->t_pflags;
-	/*
-	 * For the rolling transaction, we have to set NOFS in the old
-	 * transaction's t_pflags so that when we clear the context on
-	 * the old transaction we don't actually change the thread's NOFS
-	 * state.
-	 */
-	tp->t_pflags = current->flags | PF_MEMALLOC_NOFS;
 }
 
 #endif	/* __XFS_TRANS_H__ */
-- 
2.18.4