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Date: Tue, 31 Aug 2021 20:20:11 +0800
From: Gao Xiang <hsiangkao@linux.alibaba.com>
To: Yue Hu <zbestahu@163.com>
Subject: Re: [PATCH] erofs-utils: do not check ->idata_size for compressed
 files in erofs_prepare_inode_buffer()
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On Tue, Aug 31, 2021 at 07:56:14PM +0800, Yue Hu wrote:
> On Tue, 31 Aug 2021 19:15:50 +0800
> Gao Xiang <hsiangkao@linux.alibaba.com> wrote:

...

> > > > > > >       
> > > > > > > > 
> > > > > > > > BTW, if you have some interest, would you like to implement it? :)      
> > > > > > > 
> > > > > > > I don't know if i can finish it. But i would like to have a try :)      
> > > > > > 
> > > > > > My rough thought is to try to inline the last tail compresseed
> > > > > > extent after the on-disk compressed extents, maybe we could let
> > > > > > it work for non-compact (legacy) compress index format cases...      
> > > > > 
> > > > > I mean try to implement non-compact (legacy) compress index format cases
> > > > > first.  
> > > 
> > > I'm trying to do it under 4.19 code (since i have no 5.x environment temporarily).
> > > 
> > > Now, i think mkfs should be done. But, kernel side seems not working fine(no crash,
> > > no decompression warning/bug). Only some files are working, others not. I'm sure i
> > > can catch the inline data correctly via file dump. And I'm trying debug the issue.
> > > Maybe i need more time to read/understand more decompression code related.
> > > 
> > > BTW, now i understand no need to go z_erofs_vle_work_xxx for inline part(cur-end)
> > > , just go next_part after mapping as below, am i right? 
> > >   
> > 
> > You are right. For the common cases (except for fiemap or cases to get the exact
> > decompressed length), we only need to calculate the start of the compression extent,
> > so it's transversal in the reverse order.
> > 
> > But really... Again, I don't suggest using 4.19 staging code for real production
> > or further development. The uncompressed part is considered as stable, but
> > compression side may not (also it was disabled by default). Please also see,
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/staging/erofs/Kconfig?h=v4.19#n86
> > 
> > " config EROFS_FS_ZIP
> >   bool "EROFS Data Compresssion Support"
> >   depends on EROFS_FS
> >   help
> >     Currently we support VLE Compression only.
> >     Play at your own risk.
> > 
> >     If you don't want to use compression feature, say N. "
> > 
> > Our original first real production codebase was between 5.2~5.3. Therefore,
> > I suggest using >= 5.4 LTS codebase for production. You could also find
> > some backport codebase on github, e.g.:
> > https://github.com/nolange/erofs_kernel_4_19
> > , which backports 5.6 erofs codebase to 4.19.
> > 
> > As for tail-packing inline extent feature, how about focusing on on-disk
> > design and mkfs/erofsfuse implementation first as PoC?
> > 
> > I'm afraid that if you only focus on 4.19 codebase, the format of compact
> > indexes will be ignored, but "compact indexes" is the default option for
> > erofs now since it has less metadata overhead than non-compact indexes,
> > so both the sequential / random read are better.
> 
> OK, let me develop it under 5.4. I need taking time to find it:)

As the first step of kernel development, I think using x86 qemu should
be better since it's easier to debug than on the embedded device.

For this feature, I'm very glad to discuss some on-disk format first.
Since it's not trivial for compact indexes since it's impossible to mark
tailing-packing extent with some special blkaddr like non-compact
indexes.

My rough thought about this is "to add some new feature flag to "struct
z_erofs_map_header" and trigger z_erofs_map_blocks(i_size - 1); at a
proper time to get all information about the last tail-packing
compression extent", and when submitting io, we erofs_get_meta_page()
instead and fill the compressed pages.

But anyway, I still think focusing on mkfs.erofs and erofsfuse is a good
start for this.

Thanks,
Gao Xiang

> 
> Thanks.
> 
> > 
> > Thanks,
> > Gao Xiang
> > 
> > > Thanks.
> > >     
> > > > 
> > > > Ok, let me try to implement it.
> > > > 
> > > > Thanks.