linux-kernel - [PATCH 5.7 373/477] NFS: Fix direct WRITE throughput regression

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Date:   Tue, 23 Jun 2020 21:56:10 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Chuck Lever <chuck.lever@...cle.com>,
        Anna Schumaker <Anna.Schumaker@...app.com>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.7 373/477] NFS: Fix direct WRITE throughput regression

From: Chuck Lever <chuck.lever@...cle.com>

[ Upstream commit ba838a75e73f55a780f1ee896b8e3ecb032dba0f ]

I measured a 50% throughput regression for large direct writes.

The observed on-the-wire behavior is that the client sends every
NFS WRITE twice: once as an UNSTABLE WRITE plus a COMMIT, and once
as a FILE_SYNC WRITE.

This is because the nfs_write_match_verf() check in
nfs_direct_commit_complete() fails for every WRITE.

Buffered writes use nfs_write_completion(), which sets req->wb_verf
correctly. Direct writes use nfs_direct_write_completion(), which
does not set req->wb_verf at all. This leaves req->wb_verf set to
all zeroes for every direct WRITE, and thus
nfs_direct_commit_completion() always sets NFS_ODIRECT_RESCHED_WRITES.

This fix appears to restore nearly all of the lost performance.

Fixes: 1f28476dcb98 ("NFS: Fix O_DIRECT commit verifier handling")
Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@...app.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 fs/nfs/direct.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index a57e7c72c7f47..d49b1d1979084 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -731,6 +731,8 @@ static void nfs_direct_write_completion(struct nfs_pgio_header *hdr)
 		nfs_list_remove_request(req);
 		if (request_commit) {
 			kref_get(&req->wb_kref);
+			memcpy(&req->wb_verf, &hdr->verf.verifier,
+			       sizeof(req->wb_verf));
 			nfs_mark_request_commit(req, hdr->lseg, &cinfo,
 				hdr->ds_commit_idx);
 		}
-- 
2.25.1