Re: [PATCH v2 0/23] fs-verity support for XFS with post EOF merkle tree

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

On Wed, Jan 14, 2026 at 05:00:47AM +0000, Matthew Wilcox wrote:
> On Tue, Jan 13, 2026 at 07:45:47PM +0100, Andrey Albershteyn wrote:
> > On 2026-01-13 16:36:44, Matthew Wilcox wrote:
> > > On Mon, Jan 12, 2026 at 03:49:44PM +0100, Andrey Albershteyn wrote:
> > > > The tree is read by iomap into page cache at offset 1 << 53. This is far
> > > > enough to handle any supported file size.
> > > 
> > > What happens on 32-bit systems?  (I presume you mean "offset" as
> > > "index", so this is 1 << 65 bytes on machines with a 4KiB page size)
> > > 
> > it's in bytes, yeah I missed 32-bit systems, I think I will try to
> > convert this offset to something lower on 32-bit in iomap, as
> > Darrick suggested.
> 
> Hm, we use all 32 bits of folio->index on 32-bit plaftorms.  That's
> MAX_LFS_FILESIZE.  Are you proposing reducing that?
> 
> There are some other (performance) penalties to using 1<<53 as the lowest
> index for metadata on 64-bit.  The radix tree is going to go quite high;
> we use 6 bits at each level, so if you have a folio at 0 and a folio at
> 1<<53, you'll have a tree of height 9 and use 17 nodes.
> 
> That's going to be a lot of extra cache misses when walking the XArray
> to find any given folio.  Allowing the filesystem to decide where the
> metadata starts for any given file really is an important optimisation.
> Even if it starts at index 1<<29, you'll almost halve the number of
> nodes needed.

1<<53 is only the location of the fsverity metadata in the ondisk
mapping.  For the incore mapping, in theory we could load the fsverity
anywhere in the post-EOF part of the pagecache to save some bits.

roundup(i_size_read(), 1<<folio_max_order)) would work, right?

> Adding this ability to support RW merkel trees is certainly coming at
> a cost.  Is it worth it?  I haven't seen a user need for that articulated,
> but I haven't been paying close attention.

I think the pagecache writes of fsverity metadata are only performed
once, while enabling fsverity for a file.

--D