[PATCH] erofs-utils: add clusterofs zero check to write_uncompressed_extent()

[Gao Xiang]

Hi Yue,

On Wed, Aug 18, 2021 at 10:21:21AM +0800, Yue Hu wrote:
> Hi Xiang,
> 
> On Tue, 17 Aug 2021 22:38:04 +0800
> Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
> 
> > On Tue, Aug 17, 2021 at 10:10:46PM +0800, Yue Hu wrote:
> > > Hi Xiang,
> > > 
> > > On Tue, 17 Aug 2021 21:13:14 +0800
> > > Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
> > >   
> > > > Hi Yue,
> > > > 
> > > > On Tue, Aug 17, 2021 at 12:06:04PM +0800, Yue Hu wrote:  
> > > > > From: Yue Hu <huyue2 at yulong.com>
> > > > > 
> > > > > No need to reset clusterofs to 0 if it's already 0. Acturally, we also
> > > > > observed that case frequently. Let's avoid it.
> > > > > 
> > > > > Signed-off-by: Yue Hu <huyue2 at yulong.com>
> > > > > ---
> > > > >  lib/compress.c | 2 +-
> > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/lib/compress.c b/lib/compress.c
> > > > > index 40723a1..a8ebbc1 100644
> > > > > --- a/lib/compress.c
> > > > > +++ b/lib/compress.c
> > > > > @@ -130,7 +130,7 @@ static int write_uncompressed_extent(struct z_erofs_vle_compress_ctx *ctx,
> > > > >  	unsigned int count;
> > > > >  
> > > > >  	/* reset clusterofs to 0 if permitted */
> > > > > -	if (!erofs_sb_has_lz4_0padding() &&
> > > > > +	if (!erofs_sb_has_lz4_0padding() && ctx->clusterofs &&    
> > > > 
> > > > Also out of curiosity, which means erofs is used without lz4 0padding?  
> 
> Moreover, seems 'ctx->head' will always >= 'ctx->clusterofs' here since there
> are only 2 sites to increase it. One is 'ctx->head += count' after compression,
> another is 'ctx->head = qh_after' ( which should be same as ->clusterofs, rt?).

Sorry for some delay (since I shouldn't take working time about this.)

The design of this part is that

Due to most compression algorithms can only accept a one-shot consecutive
buffer for each lz4 block and we limit the maximal input (uncompressed)
data for each pcluster is EROFS_CONFIG_COMPR_MAX_SZ, so EROFS allocates
a buffer with the size of EROFS_CONFIG_COMPR_MAX_SZ * 2, as below:

                     2 * EROFS_CONFIG_COMPR_MAX_SZ
  _____________________________________________________________
 |_____________________________________________________________|
      ^                                     ^
      head                                  tail

head is the pending data for compression, and tail is for the next
read. As you can see, if head < EROFS_CONFIG_COMPR_MAX_SZ, we can
always get `EROFS_CONFIG_COMPR_MAX_SZ consecutive data' in this buffer.

So after head >= EROFS_CONFIG_COMPR_MAX_SZ, we needs to move head ~ tail
to the start of this buffer to keep it work.

> 
> I understand there are part of duplication data when writing uncompressed data
> to disk due to 'ctx->head' decreased?

Yeah, qh_aligned is mainly used for !0padding alignment of uncompressed
data. It's not quite useful for 0padding, since head pointer won't be
decreased.

But I don't think I like omiting ">= 'ctx->clusterofs'" (and I suspect
if it's correct or just because the special EROFS_CONFIG_COMPR_MAX_SZ)
since it involves many assumption and deep condition, and could cause
maintain burden if we change logic like this.

Thanks,
Gao Xiang

> 
> Thanks.
> 
> > > 
> > > Yes. We are using legacy compression layout now.  
> > 
> > Ok, I think sometime I will seperate it into (non)compact compress
> > indexes, and (non)0padding. It was once named as legacy just before
> > Linux < 5.3 didn't support them.
> > 
> > By default, the preferred option now I think is compact and 0padding.
> > 
> > 0padding will enable in-place decompression (and LZMA will always
> > enable it.)
> > compact will reduce metadata even further as long as the compressed
> > data is consecutive (e.g. 2 bytes each lcluster for compact indexes
> > rather than 8 bytes each lcluster for noncompact indexes, but anyway,
> > either (non)compact metadata can support effective data random access.)
> > 
> > Ok, if it's somewhat kernel 4.19 based code, I'd suggest to upgrade
> > the codebase at least to 5.4, not because 4.19 doesn't work, but
> > really for some performance concern....
> > 
> > Thanks,
> > Gao Xiang