Re: [RFC PATCH 10/11] xfs: add fs-verity support

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Dec 14, 2022 at 07:33:19AM +1100, Dave Chinner wrote:
> On Tue, Dec 13, 2022 at 11:08:45AM -0800, Eric Biggers wrote:
> > On Tue, Dec 13, 2022 at 06:29:34PM +0100, Andrey Albershteyn wrote:
> > > 
> > > Also add check that block size == PAGE_SIZE as fs-verity doesn't
> > > support different sizes yet.
> > 
> > That's coming with
> > https://lore.kernel.org/linux-fsdevel/20221028224539.171818-1-ebiggers@kernel.org/T/#u,
> > which I'll be resending soon and I hope to apply for 6.3.
> > Review and testing of that patchset, along with its associated xfstests update
> > (https://lore.kernel.org/fstests/20221211070704.341481-1-ebiggers@kernel.org/T/#u),
> > would be greatly appreciated.
> > 
> > Note, as proposed there will still be a limit of:
> > 
> > 	merkle_tree_block_size <= fs_block_size <= page_size
> 
> > Hopefully you don't need fs_block_size > page_size or
> 
> Yes, we will.
> 
> This back on my radar now that folios have settled down. It's
> pretty trivial for XFS to do because we already support metadata
> block sizes > filesystem block size. Here is an old prototype:
> 
> https://lore.kernel.org/linux-xfs/20181107063127.3902-1-david@fromorbit.com/

As per my follow-up response
(https://lore.kernel.org/r/Y5jc7P1ZeWHiTKRF@sol.localdomain),
I now think that wouldn't actually be a problem.

> > merkle_tree_block_size > fs_block_size?
> 
> That's also a desirable addition.
> 
> XFS is using xattrs to hold merkle tree blocks so the merkle tree
> storage is are already independent of the filesystem block size and
> page cache limitations. Being able to using 64kB merkle tree blocks
> would be really handy for reducing the search depth and overall IO
> footprint of really large files.

Well, the main problem is that using a Merkle tree block of 64K would mean that
you can never read less than 64K at a time.

- Eric