linux-ext4 - Re: [PATCH 4/6] fsverity: use a hashtable to find the fsverity

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Message-ID: <z4652hoxetll645hgpfuhy3pogm5y32ealgydlaz4kwve6qc2g@bl6ilzut2ybp>
Date: Mon, 19 Jan 2026 10:21:00 +0100
From: Jan Kara <jack@...e.cz>
To: Christoph Hellwig <hch@....de>
Cc: Eric Biggers <ebiggers@...nel.org>, 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>, 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 4/6] fsverity: use a hashtable to find the fsverity_info

On Mon 19-01-26 07:22:45, Christoph Hellwig wrote:
> Use the kernel's resizable hash table to find the fsverity_info.  This
> way file systems that want to support fsverity don't have to bloat
> every inode in the system with an extra pointer.  The tradeoff is that
> looking up the fsverity_info is a bit more expensive now, but the main
> operations are still dominated by I/O and hashing overhead.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

...

> diff --git a/fs/verity/enable.c b/fs/verity/enable.c
> index 95ec42b84797..91cada0d455c 100644
> --- a/fs/verity/enable.c
> +++ b/fs/verity/enable.c
> @@ -264,9 +264,24 @@ static int enable_verity(struct file *filp,
>  		goto rollback;
>  	}
>  
> +	/*
> +	 * Add the fsverity_info into the hash table before finishing the
> +	 * initialization.  This ensures we don't have to undo the enabling when
> +	 * memory allocation for the hash table fails.  This is safe because
> +	 * looking up the fsverity_info always first checks the S_VERITY flag on
> +	 * the inode, which will only be set at the very end of the
> +	 * ->end_enable_verity method.
> +	 */
> +	err = fsverity_set_info(vi);
> +	if (err)
> +		goto rollback;

OK, but since __fsverity_get_info() is just rhashtable_lookup_fast() what
prevents the CPU from reordering the hash table reads before the S_VERITY
check? I think you need a barrier in fsverity_get_info() to enforce the
proper ordering. The matching ordering during setting of S_VERITY is
implied by cmpxchg used to manipulate i_flags so that part should be fine.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR