linux-ext4 - Re: [PATCH 16/16] fsverity: use a hashtable to find the fsverity

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Message-ID: <20260127061551.GA25522@lst.de>
Date: Tue, 27 Jan 2026 07:15:51 +0100
From: Christoph Hellwig <hch@....de>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	David Sterba <dsterba@...e.com>, Theodore Ts'o <tytso@....edu>,
	Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
	Andrey Albershteyn <aalbersh@...hat.com>,
	Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net, fsverity@...ts.linux.dev
Subject: Re: [PATCH 16/16] fsverity: use a hashtable to find the
 fsverity_info

On Mon, Jan 26, 2026 at 12:40:30PM -0800, Eric Biggers wrote:
> 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.

You're right.  We allocate them, but never register them.  I'll fix
that up.

> 
> > 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()

True.  I've fixed up btrfs to avoid extra lookups.