Re: [PATCH v6 0/9] Inline Encryption Support

Satya Tangirala
2020-02-21T12:30:38Z

Hi Christoph,

I sent out v7 of the patch series. It's at
https://lore.kernel.org/linux-fscrypt/20200221115050.238976-1-satyat@google.com/T/#t

It manages keyslots on a per-request basis now - I made it get keyslots
in blk_mq_get_request, because that way I wouldn't have to worry about
programming keys in an atomic context. What do you think of the new
approach?

On Wed, Feb 05, 2020 at 10:05:41AM -0800, Christoph Hellwig wrote:
> On Tue, Feb 04, 2020 at 11:36:01PM -0800, Eric Biggers wrote:
> > The vendor-specific SMC calls do seem to work in atomic context, at least on
> > SDA845. However, in ufshcd_program_key(), the calls to pm_runtime_get_sync()
> > and ufshcd_hold() can also sleep.
> >
> > I think we can move the pm_runtime_get_sync() to ufshcd_crypto_keyslot_evict(),
> > since the block layer already ensures the device is not runtime-suspended while
> > requests are being processed (see blk_queue_enter()). I.e., keyslots can be
> > evicted independently of any bio, but that's not the case for programming them.
>
> Yes.
>
> > That still leaves ufshcd_hold(), which is still needed to ungate the UFS clocks.
> > It does accept an 'async' argument, which is used by ufshcd_queuecommand() to
> > schedule work to ungate the clocks and return SCSI_MLQUEUE_HOST_BUSY.
> >
> > So in blk_mq_dispatch_rq_list(), we could potentially try to acquire the
> > keyslot, and if it can't be done because either none are available or because
> > something else needs to be waited for, we can put the request back on the
> > dispatch list -- similar to how failure to get a driver tag is handled.
>
> Yes, that is what I had in mind.
>
> > However, if I understand correctly, that would mean that all requests to the
> > same hardware queue would be blocked whenever someone is waiting for a keyslot
> > -- even unencrypted requests and requests for unrelated keyslots.
>
> At least for an initial dumb implementation, yes. But if we care enough
> we can improve the code to check for the encrypted flag and only put
> back encrypted flags in that case.
>
> > It's possible that would still be fine for the Android use case, as vendors tend
> > to add enough keyslots to work with Android, provided that they choose the
> > fscrypt format that uses one key per encryption policy rather than one key per
> > file. I.e., it might be the case that no one waits for keyslots in practice
> > anyway. But, it seems it would be undesirable for a general Linux kernel
> > framework, which could potentially be used with per-file keys or with hardware
> > that only has a *very* small number of keyslots.
> >
> > Another option would be to allocate the keyslot in blk_mq_get_request(), where
> > sleeping is still allowed, but some merging was already done.
>
> That is another good idea. In blk_mq_get_request we acquire other
> resources like the tag, so this would be a very logical places to
> acquire the key slots. We can should also be able to still merge into
> the request while it is waiting.