Re: [PATCH v2 2/3] dm-inlinecrypt: add target for inline block device encryption

[Linlin Zhang <linlin.zhang@oss.qualcomm.com>]

On 4/28/2026 7:21 AM, Benjamin Marzinski wrote:
> On Mon, Apr 27, 2026 at 01:23:27AM -0400, Benjamin Marzinski wrote:
>> On Fri, Apr 10, 2026 at 06:40:30AM -0700, Linlin Zhang wrote:
>>> From: Eric Biggers <ebiggers@google.com>
>>> +	/*
>>> +	 * Since we've added an encryption context to the bio and
>>> +	 * blk-crypto-fallback may be needed to process it, it's necessary to
>>> +	 * use the fallback-aware bio submission code rather than
>>> +	 * unconditionally returning DM_MAPIO_REMAPPED.
>>> +	 *
>>> +	 * To get the correct accounting for a dm target in the case where
>>> +	 * __blk_crypto_submit_bio() doesn't take ownership of the bio (returns
>>> +	 * true), call __blk_crypto_submit_bio() directly and return
>>> +	 * DM_MAPIO_REMAPPED in that case, rather than relying on
>>> +	 * blk_crypto_submit_bio() which calls submit_bio() in that case.
>>> +	 */
>>> +	if (__blk_crypto_submit_bio(bio))
>>
>> This will still double account for fallback writes (which call
>> submit_bio() on the encrypted bios, and return DM_MAPIO_SUBMITTED here). 
> 
> Just to clarify, I'm talking about the vmstats accounting. The IO
> originally gets accounted by submit_bio() when the bio is submitted to
> the dm device. For actual inline encryption and fallback reads, dm will
> submit the bio to the underlying device using submit_bio_noacct() to
> avoid double-counting the IO.
> 
> For fallback writes, __blk_crypto_submit_bio() will submit the encrypted
> bios to the underlying device with submit_bio(). This adds the IO
> sectors again, even though it's the same IO, only encrypted now.


Right, thanks for calling this out.

For fallback writes, the IO is still double-counted. Given that this only
affects IO accounting in the blk-crypto fallback write slow-path and not
correctness, I think this is an acceptable tradeoff, and we can leave a
TODO to revisit the accounting once a better solution exists.

Add the bellow to the annotate.

  /*
   * TODO: blk-crypto fallback write slow-path currently double-accounts
   * IO in vmstat, as encrypted bios are submitted via submit_bio().
   * This does not affect data correctness. Consider fixing this if
   * a cleaner accounting model for derived bios is introduced.
   */

Do you agree?

> 
> -Ben
> 
>>
>> -Ben
>>
>>> +		return DM_MAPIO_REMAPPED;
>>> +	return DM_MAPIO_SUBMITTED;
>>> +}
>>
>