Re: [PATCH 2/9] btrfs: send: avoid copying file data

[Filipe Manana <fdmanana@gmail.com>]

On Fri, Aug 21, 2020 at 8:42 AM Omar Sandoval <osandov@osandov.com> wrote:
>
> From: Omar Sandoval <osandov@fb.com>
>
> send_write() currently copies from the page cache to sctx->read_buf, and
> then from sctx->read_buf to sctx->send_buf. Similarly, send_hole()
> zeroes sctx->read_buf and then copies from sctx->read_buf to
> sctx->send_buf. However, if we write the TLV header manually, we can
> copy to sctx->send_buf directly and get rid of sctx->read_buf.
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Looks good, and it passed some long duration tests with both full and
incremental sends here (with and without compression, no-holes, etc).
Only one minor thing below, but it's really subjective and doesn't
make much of a difference.

Thanks.

> ---
>  fs/btrfs/send.c | 65 +++++++++++++++++++++++++++++--------------------
>  fs/btrfs/send.h |  1 -
>  2 files changed, 39 insertions(+), 27 deletions(-)
>
> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> index 8af5e867e4ca..e70f5ceb3261 100644
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -122,8 +122,6 @@ struct send_ctx {
>
>         struct file_ra_state ra;
>
> -       char *read_buf;
> -
>         /*
>          * We process inodes by their increasing order, so if before an
>          * incremental send we reverse the parent/child relationship of
> @@ -4794,7 +4792,25 @@ static int process_all_new_xattrs(struct send_ctx *sctx)
>         return ret;
>  }
>
> -static int fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
> +static u64 max_send_read_size(struct send_ctx *sctx)

We could make this inline, since it's so small and trivial, and
constify the argument too.

> +{
> +       return sctx->send_max_size - SZ_16K;
> +}
> +
> +static int put_data_header(struct send_ctx *sctx, u32 len)
> +{
> +       struct btrfs_tlv_header *hdr;
> +
> +       if (sctx->send_max_size - sctx->send_size < sizeof(*hdr) + len)
> +               return -EOVERFLOW;
> +       hdr = (struct btrfs_tlv_header *)(sctx->send_buf + sctx->send_size);
> +       hdr->tlv_type = cpu_to_le16(BTRFS_SEND_A_DATA);
> +       hdr->tlv_len = cpu_to_le16(len);
> +       sctx->send_size += sizeof(*hdr);
> +       return 0;
> +}
> +
> +static int put_file_data(struct send_ctx *sctx, u64 offset, u32 len)
>  {
>         struct btrfs_root *root = sctx->send_root;
>         struct btrfs_fs_info *fs_info = root->fs_info;
> @@ -4804,8 +4820,11 @@ static int fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
>         pgoff_t index = offset >> PAGE_SHIFT;
>         pgoff_t last_index;
>         unsigned pg_offset = offset_in_page(offset);
> -       int ret = 0;
> -       size_t read = 0;
> +       int ret;
> +
> +       ret = put_data_header(sctx, len);
> +       if (ret)
> +               return ret;
>
>         inode = btrfs_iget(fs_info->sb, sctx->cur_ino, root);
>         if (IS_ERR(inode))
> @@ -4851,14 +4870,15 @@ static int fill_read_buf(struct send_ctx *sctx, u64 offset, u32 len)
>                 }
>
>                 addr = kmap(page);
> -               memcpy(sctx->read_buf + read, addr + pg_offset, cur_len);
> +               memcpy(sctx->send_buf + sctx->send_size, addr + pg_offset,
> +                      cur_len);
>                 kunmap(page);
>                 unlock_page(page);
>                 put_page(page);
>                 index++;
>                 pg_offset = 0;
>                 len -= cur_len;
> -               read += cur_len;
> +               sctx->send_size += cur_len;
>         }
>         iput(inode);
>         return ret;
> @@ -4880,10 +4900,6 @@ static int send_write(struct send_ctx *sctx, u64 offset, u32 len)
>
>         btrfs_debug(fs_info, "send_write offset=%llu, len=%d", offset, len);
>
> -       ret = fill_read_buf(sctx, offset, len);
> -       if (ret < 0)
> -               goto out;
> -
>         ret = begin_cmd(sctx, BTRFS_SEND_C_WRITE);
>         if (ret < 0)
>                 goto out;
> @@ -4894,7 +4910,9 @@ static int send_write(struct send_ctx *sctx, u64 offset, u32 len)
>
>         TLV_PUT_PATH(sctx, BTRFS_SEND_A_PATH, p);
>         TLV_PUT_U64(sctx, BTRFS_SEND_A_FILE_OFFSET, offset);
> -       TLV_PUT(sctx, BTRFS_SEND_A_DATA, sctx->read_buf, len);
> +       ret = put_file_data(sctx, offset, len);
> +       if (ret < 0)
> +               goto out;
>
>         ret = send_cmd(sctx);
>
> @@ -5013,8 +5031,8 @@ static int send_update_extent(struct send_ctx *sctx,
>  static int send_hole(struct send_ctx *sctx, u64 end)
>  {
>         struct fs_path *p = NULL;
> +       u64 read_size = max_send_read_size(sctx);
>         u64 offset = sctx->cur_inode_last_extent;
> -       u64 len;
>         int ret = 0;
>
>         /*
> @@ -5041,16 +5059,19 @@ static int send_hole(struct send_ctx *sctx, u64 end)
>         ret = get_cur_path(sctx, sctx->cur_ino, sctx->cur_inode_gen, p);
>         if (ret < 0)
>                 goto tlv_put_failure;
> -       memset(sctx->read_buf, 0, BTRFS_SEND_READ_SIZE);
>         while (offset < end) {
> -               len = min_t(u64, end - offset, BTRFS_SEND_READ_SIZE);
> +               u64 len = min(end - offset, read_size);
>
>                 ret = begin_cmd(sctx, BTRFS_SEND_C_WRITE);
>                 if (ret < 0)
>                         break;
>                 TLV_PUT_PATH(sctx, BTRFS_SEND_A_PATH, p);
>                 TLV_PUT_U64(sctx, BTRFS_SEND_A_FILE_OFFSET, offset);
> -               TLV_PUT(sctx, BTRFS_SEND_A_DATA, sctx->read_buf, len);
> +               ret = put_data_header(sctx, len);
> +               if (ret < 0)
> +                       break;
> +               memset(sctx->send_buf + sctx->send_size, 0, len);
> +               sctx->send_size += len;
>                 ret = send_cmd(sctx);
>                 if (ret < 0)
>                         break;
> @@ -5066,17 +5087,16 @@ static int send_extent_data(struct send_ctx *sctx,
>                             const u64 offset,
>                             const u64 len)
>  {
> +       u64 read_size = max_send_read_size(sctx);
>         u64 sent = 0;
>
>         if (sctx->flags & BTRFS_SEND_FLAG_NO_FILE_DATA)
>                 return send_update_extent(sctx, offset, len);
>
>         while (sent < len) {
> -               u64 size = len - sent;
> +               u64 size = min(len - sent, read_size);
>                 int ret;
>
> -               if (size > BTRFS_SEND_READ_SIZE)
> -                       size = BTRFS_SEND_READ_SIZE;
>                 ret = send_write(sctx, offset + sent, size);
>                 if (ret < 0)
>                         return ret;
> @@ -7145,12 +7165,6 @@ long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg)
>                 goto out;
>         }
>
> -       sctx->read_buf = kvmalloc(BTRFS_SEND_READ_SIZE, GFP_KERNEL);
> -       if (!sctx->read_buf) {
> -               ret = -ENOMEM;
> -               goto out;
> -       }
> -
>         sctx->pending_dir_moves = RB_ROOT;
>         sctx->waiting_dir_moves = RB_ROOT;
>         sctx->orphan_dirs = RB_ROOT;
> @@ -7354,7 +7368,6 @@ long btrfs_ioctl_send(struct file *mnt_file, struct btrfs_ioctl_send_args *arg)
>
>                 kvfree(sctx->clone_roots);
>                 kvfree(sctx->send_buf);
> -               kvfree(sctx->read_buf);
>
>                 name_cache_free(sctx);
>
> diff --git a/fs/btrfs/send.h b/fs/btrfs/send.h
> index ead397f7034f..de91488b7cd0 100644
> --- a/fs/btrfs/send.h
> +++ b/fs/btrfs/send.h
> @@ -13,7 +13,6 @@
>  #define BTRFS_SEND_STREAM_VERSION 1
>
>  #define BTRFS_SEND_BUF_SIZE SZ_64K
> -#define BTRFS_SEND_READ_SIZE (48 * SZ_1K)
>
>  enum btrfs_tlv_type {
>         BTRFS_TLV_U8,
> --
> 2.28.0
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”