linux-kernel - Re: [PATCH v2] btrfs: switch to library APIs for checksums

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Message-ID: <20251209194748.GH4859@twin.jikos.cz>
Date: Tue, 9 Dec 2025 20:47:49 +0100
From: David Sterba <dsterba@...e.cz>
To: Eric Biggers <ebiggers@...nel.org>
Cc: linux-btrfs@...r.kernel.org, David Sterba <dsterba@...e.com>,
	Chris Mason <clm@...com>, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v2] btrfs: switch to library APIs for checksums

On Thu, Dec 04, 2025 at 11:04:54PM -0800, Eric Biggers wrote:
> Make btrfs use the library APIs instead of crypto_shash, for all
> checksum computations.  This has many benefits:
> 
> - Allows future checksum types, e.g. XXH3 or CRC64, to be more easily
>   supported.  Only a library API will be needed, not crypto_shash too.
> 
> - Eliminates the overhead of the generic crypto layer, including an
>   indirect call for every function call and other API overhead.  A
>   microbenchmark of btrfs_check_read_bio() with crc32c checksums shows a
>   speedup from 658 cycles to 608 cycles per 4096-byte block.
> 
> - Decreases the stack usage of btrfs by reducing the size of checksum
>   contexts from 384 bytes to 240 bytes, and by eliminating the need for
>   some functions to declare a checksum context at all.
> 
> - Increases reliability.  The library functions always succeed and
>   return void.  In contrast, crypto_shash can fail and return errors.
>   Also, the library functions are guaranteed to be available when btrfs
>   is loaded; there's no longer any need to use module softdeps to try to
>   work around the crypto modules sometimes not being loaded.
> 
> - Fixes a bug where blake2b checksums didn't work on kernels booted with
>   fips=1.  Since btrfs checksums are for integrity only, it's fine for
>   them to use non-FIPS-approved algorithms.
> 
> Note that with having to handle 4 algorithms instead of just 1-2, this
> commit does result in a slightly positive diffstat.  That being said,
> this wouldn't have been the case if btrfs had actually checked for
> errors from crypto_shash, which technically it should have been doing.
> 
> Reviewed-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Eric Biggers <ebiggers@...nel.org>
> ---
> 
> v2: rebased onto latest mainline, now that both the crypto library and
>     btrfs pull requests for 6.19 have been merged

Added to for-next as we're now based on recent master, thanks.