From: Sasha Levin <sashal@kernel.org>
To: stable@vger.kernel.org
Cc: Huiwen He <hehuiwen@kylinos.cn>,
	ChenXiaoSong <chenxiaosong@kylinos.cn>,
	Steve French <stfrench@microsoft.com>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.18.y 3/4] smb/client: emulate small EOF-extending mode 0 fallocate ranges
Date: Sun,  6 Sep 2026 15:30:51 -0400
Message-ID: <20260906193052.1791917-3-sashal@kernel.org>
In-Reply-To: <20260906193052.1791917-1-sashal@kernel.org>
References: <2026090358-afar-boss-f2c3@gregkh>
 <20260906193052.1791917-1-sashal@kernel.org>
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From: Huiwen He <hehuiwen@kylinos.cn>

[ Upstream commit 7a06d3b816d73448b4e38b83d65049f090b7b201 ]

When a mode 0 fallocate extends EOF from 1G to 2G + 1M, the client
currently sends SetEOF for 2G + 1M. This can make fallocate return
success without allocating the requested range, or allocate extra
space before that range.

For example, on a fresh file:

        xfs_io -f \
          -c "falloc 0 1G" \
          -c "falloc 2G 1M" \
          -c "truncate 3G" test

The second fallocate should allocate [2G, 2G + 1M), leaving [1G, 2G)
as a hole.

Before this change, the result depended on the server allocation policy.
With Samba "strict allocate = no", SetEOF could return success without
allocating [2G, 2G + 1M). With "strict allocate = yes":

	# filefrag -v test
        [0, 1G)             allocated
        [1G, 2G)            allocated unexpectedly
        [2G, 2G + 1M)       allocated

SMB cannot allocate that arbitrary range, so write zeroes to small
EOF-extending ranges instead. Limit this to 1 MiB to bound the
client-side I/O cost.

With "strict allocate = no", the requested range [2G, 2G + 1M) is
allocated by the writes. With "strict allocate = yes":

	# filefrag -v test
        [0, 1G)             allocated
        [1G, 2G)            hole
        [2G, 2G + 1M)       allocated

This fixes the small EOF-extending range case exercised by generic/213.

Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Signed-off-by: Steve French <stfrench@microsoft.com>
Stable-dep-of: 32a7af68df73 ("cifs: add cifs_resize_file_locked() to guard fscache_resize_cookie() under i_rwsem")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/smb/client/smb2ops.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/smb/client/smb2ops.c b/fs/smb/client/smb2ops.c
index 3a833a55491d3..c25914888beae 100644
--- a/fs/smb/client/smb2ops.c
+++ b/fs/smb/client/smb2ops.c
@@ -3553,7 +3553,7 @@ static int smb3_simple_fallocate_range(unsigned int xid,
 	int rc;
 
 	buf = kvzalloc(min_t(loff_t, len, SMB2_MAX_BUFFER_SIZE), GFP_KERNEL);
-	if (buf == NULL) {
+	if (!buf) {
 		rc = -ENOMEM;
 		goto out;
 	}
-- 
2.53.0