Re: [PATCH v2 3/22] iomap: introduce IOMAP_F_BEYOND_EOF

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

On Fri, Jan 16, 2026 at 09:52:07PM +0000, Matthew Wilcox wrote:
> On Mon, Jan 12, 2026 at 03:50:05PM +0100, Andrey Albershteyn wrote:
> > Flag to indicate to iomap that read/write is happening beyond EOF and no
> > isize checks/update is needed.
> 
> Darrick and I talked to Ted about this yesterday, and we think you don't
> need the writeback portions of this.  (As Darrick said, it's hard to
> tell if this is all writeback or just some of it).
> 
> The flow for creating a verity file should be:
> 
> write data to pagecache
> calculate merkle tree data, write it to pagecache
> write_and_wait the entire file
> do verity magic
> 
> If you follow this flow, there's never a need to do writeback past EOF.
> Or indeed writeback on a verity file at all, since it's now immutable.

Hrmm, I came to rather opposite conclusions -- if you're going to write
the fsverity metadata via the pagecache then you *do* have to fix iomap
writeback to deal with post-EOF data correctly.  Right now it just
ignores everything beyond isize, and I think the xfs side of things will
still do things like try to update the ondisk size in the ioend.

I think hch or someone said that maybe those writes could be directio,
though using the pagecache for verity setup means it's all incore and
ready to go when the ioctl returns.

OTOH it would be convenient to preallocate unwritten extents for the
merkle tree, write it with directio, and now if isize ever gets
corrupted such that userspace actually /can/ read the merkle tree data,
they'll at least see zeroes.  Though I think there's an ioctl that lets
you do that anyway.

--D