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Subject: Re: [PATCH v2 0/8] ensure bios aren't split in middle of crypto data
 unit
To:     Satya Tangirala <satyat@google.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Jens Axboe <axboe@kernel.dk>,
        Eric Biggers <ebiggers@google.com>
References: <20210325212609.492188-1-satyat@google.com>
 <e0248d93-e880-6a6d-92d6-dfcfb6f9d661@acm.org> <YF07bqQ1NvPxPNrh@google.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <4694766c-8d95-1ad3-cb0c-d1ba8b7fe7ad@acm.org>
Date:   Thu, 25 Mar 2021 20:46:20 -0700
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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.

Bart.