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