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

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Dec 13, 2022 at 11:22:31AM -0800, Darrick J. Wong 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
> 
> Not right this second, but large folios (and the ability to demand them)
> are probably the last major piece that we need to support running
> 64k-fsblock filesystems on x86.
> 
> > merkle_tree_block_size > fs_block_size?
> 
> Doubtful.
> 

Thanks for the info.  Actually, I think

	merkle_tree_block_size <= fs_block_size <= page_size

is wrong.  It should actually be

	merkle_tree_block_size <= min(fs_block_size, page_size)

So there shouldn't actually be a problem with fs_block_size > page_size
(assuming that the filesystem doesn't have unrelated problems with it).

merkle_tree_block_size <= page_size comes from the way that fs/verity/verify.c
works (again, talking about the version with my patchset "fsverity: support for
non-4K pages" applied, and not the current upstream version which has a stronger
assumption of merkle_tree_block_size == page_size).

merkle_tree_block_size <= fs_block_size comes from the fact that every time data
is verified, it must be Merkle tree block aligned.  Maybe even that is not
necessarily a problem, if the filesystem waits to collect a full page (or folio)
before verifying it.  But ext4 will do a FS block at a time.

- Eric