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

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 <[email protected]> wrote:
>
> From: Huiwen He <[email protected]>
>
> 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 <[email protected]>
> Reviewed-by: ChenXiaoSong <[email protected]>
> ---
>  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
>


-- 
Thanks,

Steve