Re: [PATCH 6/9] btrfs: send: write larger chunks when using stream v2

[Josef Bacik <josef@toxicpanda.com>]

On 8/21/20 3:39 AM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> The length field of the send stream TLV header is 16 bits. This means
> that the maximum amount of data that can be sent for one write is 64k
> minus one. However, encoded writes must be able to send the maximum
> compressed extent (128k) in one command. To support this, send stream
> version 2 encodes the DATA attribute differently: it has no length
> field, and the length is implicitly up to the end of containing command
> (which has a 32-bit length field). Although this is necessary for
> encoded writes, normal writes can benefit from it, too.
> 
> For v2, let's bump up the send buffer to the maximum compressed extent
> size plus 16k for the other metadata (144k total). Since this will most
> likely be vmalloc'd (and always will be after the next commit), we round
> it up to the next page since we might as well use the rest of the page
> on systems with >16k pages.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>   fs/btrfs/send.c | 34 ++++++++++++++++++++++++++--------
>   1 file changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> index e25c3391fc02..c0f81d302f49 100644
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -4799,14 +4799,27 @@ static u64 max_send_read_size(struct send_ctx *sctx)
>   
>   static int put_data_header(struct send_ctx *sctx, u32 len)
>   {
> -	struct btrfs_tlv_header *hdr;
> +	if (sctx->flags & BTRFS_SEND_FLAG_STREAM_V2) {
> +		__le16 tlv_type;
> +
> +		if (sctx->send_max_size - sctx->send_size <
> +		    sizeof(tlv_type) + len)
> +			return -EOVERFLOW;
> +		tlv_type = cpu_to_le16(BTRFS_SEND_A_DATA);
> +		memcpy(sctx->send_buf + sctx->send_size, &tlv_type,
> +		       sizeof(tlv_type));
> +		sctx->send_size += sizeof(tlv_type);

Can we add a comment for implied length thing here?  I was reviewing this in 
vimdiff without the commit message so missed the implied length detail.  Thanks,

Josef