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Date: Fri, 23 Jul 2021 14:10:49 +0800
From: Yue Hu <zbestahu@gmail.com>
To: Gao Xiang <hsiangkao@linux.alibaba.com>
Subject: Re: [PATCH] erofs-utils: no compression case for tail-end block in
 vle_write_indexes()
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On Fri, 23 Jul 2021 13:02:14 +0800
Gao Xiang <hsiangkao@linux.alibaba.com> wrote:

> 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.

Understood.

> 
> 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

Will do it in v2.

Thanks.

> 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