RWF_DONTCACHE regression fix for xfs

[Christoph Hellwig <hch@infradead.org>]

Below is Tal's suggested fixup for xfs with the whitespace damage fixeѕ.
It should probably be attributed to Tal instead.  Can you handled it
from here and send it to Jens with your signoff/authorship to avoid
the performance regression in XFS?

From: Christoph Hellwig <hch@lst.de>
Subject: xfs: avoid double deferrals for RWF_DONTCACHE writes

XFS already defers some writes to a workqueue when transactions are
needed to process the I/O completion.  Disable the block layer bio
task completion in this case to avoid a major performance drop.

Fixes: efbde6f9f449 ("iomap: use BIO_COMPLETE_IN_TASK for dropbehind writeback")
---
 fs/xfs/xfs_aops.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index f73e26d9c9f9..059846a8d65c 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -372,10 +372,14 @@ xfs_writeback_submit(
 	}
 
 	/*
-	 * Send ioends that might require a transaction to the completion wq.
+	 * Send ioends that might require a transaction to the completion wq,
+	 * and disable the block layer task completion for them as there is no
+	 * need to defer twice.
 	 */
-	if (xfs_ioend_needs_wq_completion(ioend))
+	if (xfs_ioend_needs_wq_completion(ioend)) {
 		ioend->io_bio.bi_end_io = xfs_end_bio;
+		bio_clear_flag(&ioend->io_bio, BIO_COMPLETE_IN_TASK);
+	}
 
 	return iomap_ioend_writeback_submit(wpc, error);
 }
@@ -481,7 +485,14 @@ xfs_zoned_writeback_submit(
 {
 	struct iomap_ioend		*ioend = wpc->wb_ctx;
 
+	/*
+	 * Defer all completions to our workqueue as all zoned writes require a
+	 * transaction to be persisted.  This also means we never need the block
+	 * layer in-task completion for a task context.
+	 */
 	ioend->io_bio.bi_end_io = xfs_end_bio;
+	bio_clear_flag(&ioend->io_bio, BIO_COMPLETE_IN_TASK);
+
 	if (error) {
 		ioend->io_bio.bi_status = errno_to_blk_status(error);
 		bio_endio(&ioend->io_bio);
-- 
2.53.0