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

[Matthew Wilcox <willy@infradead.org>]

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.

> > +	/* 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.