linux-kernel - Re: [PATCH v7] iomap: make inline data support more flexible

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Message-ID: <YP6vs180ThT1A2dO@B-P7TQMD6M-0146.local>
Date:   Mon, 26 Jul 2021 20:50:59 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Andreas Grünbacher 
        <andreas.gruenbacher@...il.com>
Cc:     Christoph Hellwig <hch@....de>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Huang Jianan <huangjianan@...o.com>,
        linux-erofs@...ts.ozlabs.org,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7] iomap: make inline data support more flexible

Hi Andreas, Christoph,

On Mon, Jul 26, 2021 at 02:27:12PM +0200, Andreas Grünbacher wrote:
> Am Mo., 26. Juli 2021 um 14:17 Uhr schrieb Christoph Hellwig <hch@....de>:
> >
> > > Subject: iomap: Support tail packing
> >
> > I can't say I like this "tail packing" language here when we have the
> > perfectly fine inline wording.  Same for various comments in the actual
> > code.
> >
> > > +     /* inline and tail-packed data must start page aligned in the file */
> > > +     if (WARN_ON_ONCE(offset_in_page(iomap->offset)))
> > > +             return -EIO;
> > > +     if (WARN_ON_ONCE(size > PAGE_SIZE - offset_in_page(iomap->inline_data)))
> > > +             return -EIO;
> >
> > Why can't we use iomap_inline_data_size_valid here?
> 
> We can now. Gao, can you change that?

Thank you all taking so much time on this! much appreciated.

I'm fine to update that.

> 
> > That is how can size be different from iomap->length?
> 
> Quoting from my previous reply,
> 
> "In the iomap_readpage case (iomap_begin with flags == 0),
> iomap->length will be the amount of data up to the end of the inode.

For tail-packing cases, iomap->length is just the length of tail-packing
inline extent.

> In the iomap_file_buffered_write case (iomap_begin with flags ==
> IOMAP_WRITE), iomap->length will be the size of iomap->inline_data.
> (For extending writes, we need to write beyond the current end of
> inode.) So iomap->length isn't all that useful for
> iomap_read_inline_data."

Ok, now it seems I get your point. For the current gfs2 inline cases:
  iomap_write_begin
    iomap_write_begin_inline
      iomap_read_inline_data

here, gfs2 passes a buffer instead with "iomap->length", maybe it
could be larger than i_size_read(inode) for gfs2. Is that correct?

	loff_t max_size = gfs2_max_stuffed_size(ip);

	iomap->length = max_size;

If that is what gfs2 currently does, I think it makes sense to
temporarily use as this, but IMO, iomap->inline_bufsize is not
iomap->length. These are 2 different concepts.

> 
> > Shouldn't the offset_in_page also go into iomap_inline_data_size_valid,
> > which should probably be called iomap_inline_data_valid then?
> 
> Hmm, not sure what you mean: iomap_inline_data_size_valid does take
> offset_in_page(iomap->inline_data) into account.
> 
> > >       if (iomap->type == IOMAP_INLINE) {
> > > +             int ret = iomap_read_inline_data(inode, page, iomap);
> > > +             return ret ?: PAGE_SIZE;
>
> > The ?: expression without the first leg is really confuing.  Especially
> > if a good old if is much more readable here.
> 
> I'm sure Gao can change this.
> 
> >                 int ret = iomap_read_inline_data(inode, page, iomap);
> >
> >                 if (ret)
> >                         return ret;
> >                 return PAGE_SIZE;

I'm fine to update it if no strong opinion.

> >
> > > +             copied = copy_from_iter(iomap_inline_data(iomap, pos), length, iter);
> >
> >
> > > +             copied = copy_to_iter(iomap_inline_data(iomap, pos), length, iter);
> >
> > Pleae avoid the overly long lines.
> 
> I thought people were okay with 80 character long lines?

Christoph mentioned before as below:
https://lore.kernel.org/linux-fsdevel/YPVe41YqpfGLNsBS@infradead.org/

We also need to take the offset into account for the write side.
I guess it would be nice to have a local variable for the inline
address to not duplicate that calculation multiple times.

Thanks,
Gao Xiang

> 
> Thanks,
> Andreas