linux-kernel - Re: [PATCH v4] iomap: support tail packing inline read

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Date:   Wed, 21 Jul 2021 10:53:58 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>
Cc:     "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-erofs@...ts.ozlabs.org,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4] iomap: support tail packing inline read

Hi Andreas,

On Wed, Jul 21, 2021 at 04:26:47AM +0200, Andreas Grünbacher wrote:
> Am Mi., 21. Juli 2021 um 02:33 Uhr schrieb Gao Xiang
> <hsiangkao@...ux.alibaba.com>:
> > > And since you can only kmap one page at a time, an inline read grabs the
> > > first part of the data in "page one" and then we have to call
> > > iomap_begin a second time get a new address so that we can read the rest
> > > from "page two"?
> >
> > Nope, currently EROFS inline data won't cross page like this.
> >
> > But in principle, yes, I don't want to limit it to the current
> > EROFS or gfs2 usage. I think we could make this iomap function
> > more generally (I mean, I'd like to make the INLINE extent
> > functionity as general as possible,
> 
> Nono. Can we please limit this patch what we actually need right now,
> and worry about extending it later?

Can you elaborate what it will benefit us if we only support one tail
block for iomap_read_inline_data()? (I mean it has similar LOC changes,
similar implementation / complexity.) The only concern I think is if
it causes gfs2 regression, so that is what I'd like to confirm.

In contrast, I'd like to avoid iomap_write_begin() tail-packing because
it's complex and no fs user interests in it for now. So I leave it
untouched for now.

Another concern I really like to convert EROFS to iomap is I'd like to
support sub-page blocksize for EROFS after converting. I don't want to
touch iomap inline code again like this since it interacts 2 directories
thus cause too much coupling.

Thanks,
Gao Xiang

> 
> > my v1 original approach
> > in principle can support any inline extent in the middle of
> > file rather than just tail blocks, but zeroing out post-EOF
> > needs another iteration) and I don't see it add more code and
> > complexity.
> 
> Thanks,
> Andreas