On 2020/06/24 14:05, Eric Biggers wrote:
> On Fri, Jun 19, 2020 at 05:41:32PM +0100, Ignat Korchagin wrote:
>> Sometimes extra thread offloading imposed by dm-crypt hurts IO latency. This is
>> especially visible on busy systems with many processes/threads. Moreover, most
>> Crypto API implementaions are async, that is they offload crypto operations on
>> their own, so this dm-crypt offloading is excessive.
>
> This really should say "some Crypto API implementations are async" instead of
> "most Crypto API implementations are async".
>
> Notably, the AES-NI implementation of AES-XTS is synchronous if you call it in a
> context where SIMD instructions are usable. It's only asynchronous when SIMD is
> not usable. (This seems to have been missed in your blog post.)
>
>> This adds a new flag, which directs dm-crypt not to offload crypto operations
>> and process everything inline. For cases, where crypto operations cannot happen
>> inline (hard interrupt context, for example the read path of the NVME driver),
>> we offload the work to a tasklet rather than a workqueue.
>
> This patch both removes some dm-crypt specific queueing, and changes decryption
> to use softIRQ context instead of a workqueue. It would be useful to know how
> much of a difference the workqueue => softIRQ change makes by itself. Such a
> change could be useful for fscrypt as well. (fscrypt uses a workqueue for
> decryption, but besides that doesn't use any other queueing.)
>
>> @@ -127,7 +128,7 @@ struct iv_elephant_private {
>> * and encrypts / decrypts at the same time.
>> */
>> enum flags { DM_CRYPT_SUSPENDED, DM_CRYPT_KEY_VALID,
>> - DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD };
>> + DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD, DM_CRYPT_FORCE_INLINE = (sizeof(unsigned long) * 8 - 1) };
>
> Assigning a specific enum value isn't necessary.
>
>> @@ -1458,13 +1459,18 @@ static void crypt_alloc_req_skcipher(struct crypt_config *cc,
>>
>> skcipher_request_set_tfm(ctx->r.req, cc->cipher_tfm.tfms[key_index]);
>>
>> - /*
>> - * Use REQ_MAY_BACKLOG so a cipher driver internally backlogs
>> - * requests if driver request queue is full.
>> - */
>> - skcipher_request_set_callback(ctx->r.req,
>> - CRYPTO_TFM_REQ_MAY_BACKLOG,
>> - kcryptd_async_done, dmreq_of_req(cc, ctx->r.req));
>> + if (test_bit(DM_CRYPT_FORCE_INLINE, &cc->flags))
>> + /* make sure we zero important fields of the request */
>> + skcipher_request_set_callback(ctx->r.req,
>> + 0, NULL, NULL);
>> + else
>> + /*
>> + * Use REQ_MAY_BACKLOG so a cipher driver internally backlogs
>> + * requests if driver request queue is full.
>> + */
>> + skcipher_request_set_callback(ctx->r.req,
>> + CRYPTO_TFM_REQ_MAY_BACKLOG,
>> + kcryptd_async_done, dmreq_of_req(cc, ctx->r.req));
>> }
>
> This looks wrong. Unless type=0 and mask=CRYPTO_ALG_ASYNC are passed to
> crypto_alloc_skcipher(), the skcipher implementation can still be asynchronous,
> in which case providing a callback is required.
>
> Do you intend that the "force_inline" option forces the use of a synchronous
> skcipher (alongside the other things it does)? Or should it still allow
> asynchronous ones?
>
> We may not actually have a choice in that matter, since xts-aes-aesni has the
> CRYPTO_ALG_ASYNC bit set (as I mentioned) despite being synchronous in most
> cases; thus, the crypto API won't give you it if you ask for a synchronous
> cipher. So I think you still need to allow async skciphers? That means a
> callback is still always required.
Arg... So it means that some skciphers will not be OK at all for SMR writes. I
was not aware of these differences (tested with aes-xts-plain64 only). The ugly
way to support async ciphers would be to just wait inline for the crypto API to
complete using a completion for instance. But that is very ugly. Back to
brainstorming, and need to learn more about the crypto API...
>
> - Eric
>
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>
--
Damien Le Moal
Western Digital Research
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