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

[hehuiwen <huiwen.he@linux.dev>]

Hi steve,

The normal write path can use larger writes, and dd is a good example of
that. But this patch is only changing the fallocate zero-write emulation
helper.

That helper already capped each SMB2_write to SMB2_MAX_BUFFER_SIZE 
before this patch:

        if (io_parms.length > SMB2_MAX_BUFFER_SIZE)
                io_parms.length = SMB2_MAX_BUFFER_SIZE;

So this patch does not reduce the write size on the wire. It only avoids
allocating a 1 MiB zero buffer when the current helper sends at most 64 
KiB per write.

Using the negotiated wsize here could be a useful performance 
optimization, but it needs more care to make sure the larger
write path is handled safely.

Thanks,
Huiwen

在 2026/7/2 05:44, Steve French 写道:
> Won't this hurt performance since for most servers the default max
> write size is 4MB (e.g. Samba or Windows) or 1MB?  I did a quick
> experiment with
> "dd" and I see the 4MB writes over the wire not 64K
> 
> On Wed, Jul 1, 2026 at 10:22 AM Huiwen He <huiwen.he@linux.dev> wrote:
>>
>> 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
>>
> 
>