[PATCH v3] iomap: support tail packing inline read

[Gao Xiang]

On Mon, Jul 19, 2021 at 05:31:10PM +0100, Matthew Wilcox wrote:
> On Tue, Jul 20, 2021 at 12:11:49AM +0800, Gao Xiang wrote:
> > On Mon, Jul 19, 2021 at 05:13:10PM +0200, Christoph Hellwig wrote:
> > > On Mon, Jul 19, 2021 at 04:02:30PM +0100, Matthew Wilcox wrote:
> > > > > +	if (iomap->type == IOMAP_INLINE) {
> > > > > +		iomap_read_inline_data(inode, page, iomap, pos);
> > > > > +		plen = PAGE_SIZE - poff;
> > > > > +		goto done;
> > > > > +	}
> > > > 
> > > > This is going to break Andreas' case that he just patched to work.
> > > > GFS2 needs for there to _not_ be an iop for inline data.  That's
> > > > why I said we need to sort out when to create an iop before moving
> > > > the IOMAP_INLINE case below the creation of the iop.
> > > > 
> > > > If we're not going to do that first, then I recommend leaving the
> > > > IOMAP_INLINE case where it is and changing it to ...
> > > > 
> > > > 	if (iomap->type == IOMAP_INLINE)
> > > > 		return iomap_read_inline_data(inode, page, iomap, pos);
> > > > 
> > > > ... and have iomap_read_inline_data() return the number of bytes that
> > > > it copied + zeroed (ie PAGE_SIZE - poff).
> > > 
> > > Returning the bytes is much cleaner anyway.  But we still need to deal
> > > with the sub-page uptodate status in one form or another.
> > 
> > There is another iomap_read_inline_data() caller as in:
> > +static int iomap_write_begin_inline(struct inode *inode, loff_t pos,
> > +		struct page *page, struct iomap *srcmap)
> > +{
> > +	/* needs more work for the tailpacking case, disable for now */
> > +	if (WARN_ON_ONCE(pos != 0))
> > +		return -EIO;
> > +	if (PageUptodate(page))
> > +		return 0;
> > +	iomap_read_inline_data(inode, page, srcmap, pos);
> > +	return 0;
> > +}
> > 
> > I'd like to avoid it as (void)iomap_read_inline_data(...). That's why it
> > left as void return type.
> 
> You don't need to cast away the return value in C.

Well, I don't check the current behavior of this, but see:
http://c-faq.com/style/voidcasts.html

Anyway, that's minor and easy to update if really needed.
I'd like to check if it works first...

Thanks,
Gao Xiang