Re: [PATCH 5/9] blk-crypto: optimize bio splitting in blk_crypto_fallback_encrypt_bio

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Dec 10, 2025 at 04:23:34PM +0100, Christoph Hellwig wrote:
> The current code in blk_crypto_fallback_encrypt_bio is inefficient and
> prone to deadlocks under memory pressure: It first walks the passed in
> plaintext bio to see how much of it can fit into a single encrypted
> bio using up to BIO_MAX_VEC PAGE_SIZE segments, and then allocates a
> plaintext clone that fits the size, only to allocate another bio for
> the ciphertext later.  While the plaintext clone uses a bioset to avoid
> deadlocks when allocations could fail, the ciphertex one uses bio_kmalloc
> which is a no-go in the file system I/O path.
> 
> Switch blk_crypto_fallback_encrypt_bio to walk the source plaintext bio
> while consuming bi_iter without cloning it, and instead allocate a
> ciphertext bio at the beginning and whenever we fille up the previous
> one.  The existing bio_set for the plaintext clones is reused for the
> ciphertext bios to remove the deadlock risk.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  block/blk-crypto-fallback.c | 164 +++++++++++++++---------------------
>  1 file changed, 66 insertions(+), 98 deletions(-)

Reviewed-by: Eric Biggers <ebiggers@kernel.org>

- Eric