Re: [PATCH v2 0/8] ensure bios aren't split in middle of crypto data unit

From: Satya Tangirala <satyat@google.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jens Axboe <axboe@kernel.dk>, Eric Biggers <ebiggers@google.com>
Subject: Re: [PATCH v2 0/8] ensure bios aren't split in middle of crypto data unit
Date: Fri, 26 Mar 2021 07:56:16 +0000	[thread overview]
Message-ID: <YF2ToMDxIpVpKftI@google.com> (raw)
In-Reply-To: <4694766c-8d95-1ad3-cb0c-d1ba8b7fe7ad@acm.org>

On Thu, Mar 25, 2021 at 08:46:20PM -0700, Bart Van Assche wrote:
> On 3/25/21 6:39 PM, Satya Tangirala wrote:
> > On Thu, Mar 25, 2021 at 02:51:31PM -0700, Bart Van Assche wrote:
> >> Are you sure that the block layer core splits bios at logical block
> >> boundaries? Commit 9cc5169cd478 ("block: Improve physical block
> >> alignment of split bios") should have changed the behavior from
> >> splitting at logical block boundaries into splitting at physical block
> >> boundaries.
> >
> > Ah, what I really meant with that sentence was "Currently, if a bio is
> > split, the size of the resulting bio is guaranteed to be aligned to a
> > the lbs. The endpoint of the bio might also be aligned to a physical
> > block boundary (which 9cc5169cd478 tries to achieve if possible), but
> > the bio's size (and hence also its endpoint since the start of the bio
> > is always lbs aligned) is *at least* lbs aligned". Does that sound
> > accurate?
> That sounds better to me :-)
> 
> >> Without having looked at this patch series, can the same
> >> effect be achieved by reporting the crypto data unit size as the
> >> physical block size?
> >
> > That would've been awesome, but I don't think we can do that :(
> > 1) There isn't only one crypto data unit size. A device can support,
> >    and upper layers are free to use, many different data unit sizes.
> > 2) IIUC 9cc5169cd478 (or more accurately get_max_io_size() since the
> >    function has been changed slightly since your original patch)
> >    doesn't align the size of the bio to the pbs - it only aligns the
> >    endpoint of the bio to the pbs (and it may actually not even do
> >    that if it turns out to not be possible). Is that right? If so,
> >    that means that if the startpoint of the bio isn't pbs aligned, the
> >    size of the bio won't be pbs aligned either.
> 
> Hmm ... if the start of a bio is not aligned to the physical block size
> I don't think that the block layer can do anything about the start of
> the bio. Anyway, I have taken a quick look at this patch series and the
> patch series looks pretty clean to me. I will let Christoph review this
> patch series since he already reviewed the previous version of this series.
Sounds good. Thanks for looking through the series!
> 
> Bart.