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

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Message-ID: <CAHc6FU6t84w-RoV7jyQmVAdvjEk6AcVVL6YkhBr10rNKs8DOrQ@mail.gmail.com>
Date:   Mon, 26 Jul 2021 15:10:51 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Andreas Grünbacher <andreas.gruenbacher@...il.com>,
        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

On Mon, Jul 26, 2021 at 2:51 PM Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:
> 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.

Fair enough, we could add a local variable:

  void *inline_data = iomap_inline_data(iomap, pos);

and use that in the copy_from_iter and copy_to_iter. Why not.

Thanks,
Andreas