Re: [PATCH] erofs-utils: no compression case for tail-end block in vle_write_indexes()

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Yue,

On Fri, Jul 23, 2021 at 11:49:45AM +0800, Yue Hu wrote:
> From: Yue Hu <huyue2@yulong.com>
> 
> Note that count value will be always greater than EROFS_BLKSIZ when
> calling erofs_compress_destsize() in vle_compress_one(). So, the d1
> always >= 1 for compressed block in vle_write_indexes(). That is to
> say tail-end block can't be compressed.
> 
> Signed-off-by: Yue Hu <huyue2@yulong.com>

I once thought tail end block can be compressed with < EROFS_BLKSIZ as
well (as long as it has some benefit) and also tail-packing inline
like uncompressed cases together after compress extents, so that when we
reading the last compress extent meta block, the tail-packing compressed
data can be loaded together.

Currently, I think we could do that, but could you add some DBG_BUGON
like DBG_BUGON(!raw) right above
"type = Z_EROFS_VLE_CLUSTER_TYPE_PLAIN;" and a TODO comment to mention
tail-packing inline compressed cases is under TODO list? That would be
much better.

Thanks,
Gao Xiang

> ---
>  lib/compress.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/compress.c b/lib/compress.c
> index af0c720..93fc543 100644
> --- a/lib/compress.c
> +++ b/lib/compress.c
> @@ -73,10 +73,9 @@ static void vle_write_indexes(struct z_erofs_vle_compress_ctx *ctx,
>  
>  	di.di_clusterofs = cpu_to_le16(ctx->clusterofs);
>  
> -	/* whether the tail-end (un)compressed block or not */
> +	/* whether the tail-end uncompressed block or not */
>  	if (!d1) {
> -		type = raw ? Z_EROFS_VLE_CLUSTER_TYPE_PLAIN :
> -			Z_EROFS_VLE_CLUSTER_TYPE_HEAD;
> +		type = Z_EROFS_VLE_CLUSTER_TYPE_PLAIN;
>  		advise = cpu_to_le16(type << Z_EROFS_VLE_DI_CLUSTER_TYPE_BIT);
>  
>  		di.di_advise = advise;
> -- 
> 1.9.1