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

[Benjamin Marzinski <bmarzins@redhat.com>]

On Wed, Apr 29, 2026 at 08:34:00PM +0800, Linlin Zhang wrote:
> 
> 
> On 4/29/2026 12:36 AM, Benjamin Marzinski wrote:
> > On Tue, Apr 28, 2026 at 05:20:07PM +0800, Linlin Zhang wrote:
> >>
> >>
> >> 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?
> > 
> > You could add an extra argument, for instance "bool need_acct", to
> > __blk_crypto_submit_bio(), and plumb it through to
> > __blk_crypto_fallback_encrypt_bio(), where it could be used to choose
> > between calling submit_bio() and and submit_bio_noacct().
> > 
> > We could even add a flag to cloned bios for stacked devices, that could
> > be checked in submit_bio(), so we didn't need to have
> > submit_bio_noacct(). But this is a pretty niche case with other
> > solutions, so I'm not sure if it warrants adding more checks to
> > submit_bio().
> > 
> > I do agree that people probably aren't using dm-inlinecrypt for devices
> > where they don't actually have inline encryption capabilities, so it's
> > not a major issue. What to you think, Mikulas?
> 
> Thanks for the suggestions.
> 
> Adding a bool need_acct parameter to __blk_crypto_submit_bio() would require
> updating all existing callers, which feels rather intrusive given that the
> accounting issue only affects the blk‑crypto fallback write slow‑path. I’m a
> bit concerned that this would broaden the scope of the change more than
> necessary for the problem at hand.

I get your concern, and I'd like a second opinion on how much we should
care about this, but it doesn't look like there are many other callers
that would be effected here. The only existing caller of
__blk_crypto_submit_bio() is blk_crypto_submit_bio(), which would just
call it with "need_acct=true". Looking at the code path below
__blk_crypto_submit_bio() that would need to change for submitting the
bios:

__blk_crypto_submit_bio() is the only caller of
blk_crypto_fallback_bio_prep()

blk_crypto_fallback_bio_prep() is the only caller of
blk_crypto_fallback_encrypt_bio().

blk_crypto_fallback_encrypt_bio() is the only caller of
__blk_crypto_fallback_encrypt_bio(), which is the function that would
need to choose between submit_bio() and submit_bio_noacct().

Doing this would change the crypto API (by necessity, since we're adding
a new argument to __blk_crypto_submit_bio() for stacking devices to
use), and it is adds a extra argument to a number of functions, just to
handle this corner case. But it is still a relatively contained change.

-Ben