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

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Jul 21, 2021 at 08:03:44AM +0800, Gao Xiang wrote:
> On Tue, Jul 20, 2021 at 10:18:54PM +0100, Matthew Wilcox wrote:
> > On Tue, Jul 20, 2021 at 01:42:24PM -0700, Darrick J. Wong wrote:
> > > > -	BUG_ON(page_has_private(page));
> > > > -	BUG_ON(page->index);
> > > > -	BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > > > +	/* inline source data must be inside a single page */
> > > > +	BUG_ON(iomap->length > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > > 
> > > Can we reduce the strength of these checks to a warning and an -EIO
> > > return?
> > 
> > I'm not entirely sure that we need this check, tbh.
> 
> I'm fine to get rid of this check, it just inherited from:
>  - BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
> 
> It has no real effect, but when reading INLINE extent, its .iomap_begin()
> does:
> 	iomap->private = erofs_get_meta_page()	/* get meta page */
> 
> and in the .iomap_end(), it does:
> 	struct page *ipage = iomap->private;
> 	if (ipage) {
> 		unlock_page(ipage);
> 		put_page(ipage);
> 	}
> 
> > 
> > > > +	/* handle tail-packing blocks cross the current page into the next */
> > > > +	size = min_t(unsigned int, iomap->length + pos - iomap->offset,
> > > > +		     PAGE_SIZE - poff);
> > > >  
> > > >  	addr = kmap_atomic(page);
> > > > -	memcpy(addr, iomap->inline_data, size);
> > > > -	memset(addr + size, 0, PAGE_SIZE - size);
> > > > +	memcpy(addr + poff, iomap->inline_data - iomap->offset + pos, size);
> > > > +	memset(addr + poff + size, 0, PAGE_SIZE - poff - size);
> > > 
> > > Hmm, so I guess the point of this is to support reading data from a
> > > tail-packing block, where each file gets some arbitrary byte range
> > > within the tp-block, and the range isn't aligned to an fs block?  Hence
> > > you have to use the inline data code to read the relevant bytes and copy
> > > them into the pagecache?
> > 
> > I think there are two distinct cases for IOMAP_INLINE.  One is
> > where the tail of the file is literally embedded into the inode.
> > Like ext4 fast symbolic links.  Taking the ext4 i_blocks layout
> > as an example, you could have a 4kB block stored in i_block[0]
> > and then store bytes 4096-4151 in i_block[1-14] (although reading
> > https://www.kernel.org/doc/html/latest/filesystems/ext4/dynamic.html
> > makes me think that ext4 only supports storing 0-59 in the i_blocks;
> > it doesn't support 0-4095 in i_block[0] and then 4096-4151 in i_blocks)
> > 
> > The other is what I think erofs is doing where, for example, you'd
> > specify in i_block[1] the block which contains the tail and then in
> > i_block[2] what offset of the block the tail starts at.
> 
> Nope, EROFS inline data is embedded into the inode in order to save
> I/O as well as space (maybe I didn't express clear before [1]). 
> 
> I understand the other one, but it can only save storage space but
> cannot save I/O (we still need another independent I/O to read its
> meta buffered page).
> 
> In the view of INLINE extent itself, I think both ways can be
> supported with this approach.

OH, I see, so you need the multi-page inline data support because the
ondisk layout is something like this:

+----------- page one ---------+----------- page two...
V                              V
+-------+-----------------------------+---------
| inode |   inline data               | inode...
+-------+-----------------------------+---------

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"?

--D

> 
> [1] https://www.kernel.org/doc/html/latest/filesystems/erofs.html
>     "On-disk details" section.
> 
> Thanks,
> Gao Xiang