linux-ext4 - [PATCHv4 1/3] ext4: fix ext4_dax_read/write inode locking sequence for IOCB

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Date:   Thu,  5 Dec 2019 12:16:22 +0530
From:   Ritesh Harjani <riteshh@...ux.ibm.com>
To:     jack@...e.cz, tytso@....edu, linux-ext4@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, mbobrowski@...browski.org,
        riteshh@...ux.ibm.com, joseph.qi@...ux.alibaba.com
Subject: [PATCHv4 1/3] ext4: fix ext4_dax_read/write inode locking sequence for  IOCB_NOWAIT

Apparently our current rwsem code doesn't like doing the trylock, then
lock for real scheme.  So change our dax read/write methods to just do the
trylock for the RWF_NOWAIT case.
This seems to fix AIM7 regression in some scalable filesystems upto ~25%
in some cases. Claimed in commit 942491c9e6d6 ("xfs: fix AIM7 regression")

Reviewed-by: Jan Kara <jack@...e.cz>
Reviewed-by: Matthew Bobrowski <mbobrowski@...browski.org>
Signed-off-by: Ritesh Harjani <riteshh@...ux.ibm.com>
---
 fs/ext4/file.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 6a7293a5cda2..977ac58dc718 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -88,9 +88,10 @@ static ssize_t ext4_dax_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	struct inode *inode = file_inode(iocb->ki_filp);
 	ssize_t ret;
 
-	if (!inode_trylock_shared(inode)) {
-		if (iocb->ki_flags & IOCB_NOWAIT)
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!inode_trylock_shared(inode))
 			return -EAGAIN;
+	} else {
 		inode_lock_shared(inode);
 	}
 	/*
@@ -487,9 +488,10 @@ ext4_dax_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	bool extend = false;
 	struct inode *inode = file_inode(iocb->ki_filp);
 
-	if (!inode_trylock(inode)) {
-		if (iocb->ki_flags & IOCB_NOWAIT)
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!inode_trylock(inode))
 			return -EAGAIN;
+	} else {
 		inode_lock(inode);
 	}
 
-- 
2.21.0