[PATCH v6 2/5] smb/client: reduce fallocate zero buffer allocation

[Huiwen He <huiwen.he@linux.dev>]

From: Huiwen He <hehuiwen@kylinos.cn>

The fallocate emulation allocates a 1 MiB zero-filled buffer even
though each SMB2_write request is limited to SMB2_MAX_BUFFER_SIZE,
which is 64 KiB. A high-order 1 MiB allocation is more likely to
fail on a fragmented system.

Allocate only the smaller of the requested range and SMB2_MAX_BUFFER_SIZE,
and reuse that zero-filled buffer for every write request. Also reject
a successful write that makes no progress to avoid looping indefinitely.

This reduces the contiguous allocation required by fallocate emulation
without changing the written data or range semantics.

Signed-off-by: Huiwen He <hehuiwen@kylinos.cn>
Reviewed-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
---
 fs/smb/client/smb2ops.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/smb/client/smb2ops.c b/fs/smb/client/smb2ops.c
index cc8e0595e504..23505ae9bd81 100644
--- a/fs/smb/client/smb2ops.c
+++ b/fs/smb/client/smb2ops.c
@@ -3559,7 +3559,7 @@ static int smb3_simple_fallocate_write_range(unsigned int xid,
 					     char *buf)
 {
 	struct cifs_io_parms io_parms = {0};
-	int nbytes;
+	unsigned int nbytes;
 	int rc = 0;
 	struct kvec iov[2];
 
@@ -3580,9 +3580,10 @@ static int smb3_simple_fallocate_write_range(unsigned int xid,
 		rc = SMB2_write(xid, &io_parms, &nbytes, iov, 1);
 		if (rc)
 			break;
+		if (!nbytes)
+			return -EIO;
 		if (nbytes > len)
 			return -EINVAL;
-		buf += nbytes;
 		off += nbytes;
 		len -= nbytes;
 	}
@@ -3611,7 +3612,7 @@ static int smb3_simple_fallocate_range(unsigned int xid,
 	if (rc)
 		goto out;
 
-	buf = kzalloc(1024 * 1024, GFP_KERNEL);
+	buf = kzalloc(min_t(loff_t, len, SMB2_MAX_BUFFER_SIZE), GFP_KERNEL);
 	if (buf == NULL) {
 		rc = -ENOMEM;
 		goto out;
-- 
2.43.0