Re: [PATCH 16/16] fsverity: use a hashtable to find the fsverity_info

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Jan 26, 2026 at 05:51:02AM +0100, Christoph Hellwig wrote:
> The file open path uses rhashtable_lookup_get_insert_fast,
> which can either find an existing object for the hash key or insert a
> new one in a single atomic operation, so that concurrent opens never
> allocate duplicate fsverity_info structure.

They still do, though.  But that's unchanged from before.
ensure_verity_info() frees the one it allocated if it finds that one got
set concurrently.

> Because insertion into the hash table now happens before S_VERITY is set,
> fsverity just becomes a barrier and a flag check and doesn't have to look
> up the fsverity_info at all, so there is only a single lookup per
> ->read_folio or ->readahead invocation.  For btrfs there is an additional
> one for each bio completion, while for ext4 and f2fs the fsverity_info
> is stored in the per-I/O context and reused for the completion workqueue.

btrfs actually still looks up the verity info once per folio.  See:

    btrfs_readahead()
        -> btrfs_do_readpage()
            -> fsverity_get_info()

- Eric