[dm-devel] CFQ and dm-crypt

[Milan Broz]

On 10/25/2010 11:53 AM, Richard Kralovic wrote:

> Do you think it is possible to handle this in device-mapper, without any
> support from the cfq code?
> 
> I also noticed that a solution for this problem was proposed a few years
> ago by Hirokazu Takahashi (a patch for linux-2.6.25,
> http://lkml.org/lkml/2008/4/22/193), but there was no response to it. Is
> such an approach wrong?

Not sure but it will be discussed now.
There are more situations where the process id is lost because of other queue,
a lot of new recent code (including internal thread in dm-core).

>>> Other possibility is to avoid using separate threads for doing io in dm
>>> modules. The attached patch (against 2.6.36) modifies dm-crypt in this
>>> way, what results into much better behavior of cfq (e.g., io priorities
>>> work correctly).
>>
>> Sorry, this completely dismantles the way how dm-crypt solves problems
>> with stacking dm devices.
>> Basically it reintroduces possible deadlocks for low memory
>> situations (the reason why there are these threads).
> 
> Would the problem with deadlock be still present if the io worker queue
> was used for writes only, but reads were issued directly? (Even this
> would be a significant improvement for people using cfq and a full-disk
> encryption over dm-crypt, since asynchronous writes are not supported by
> cfq anyway.)

Sorry, both must be issued from separate thread, you must not block in
common crypt_map() call:

- READ must first allocate BIO clone for ciphertext data
(it can wait here - memory allocation, imagine it waits for swap -> swap is
on another crypt device -> deadlock)

- WRITES first run encryption thus must allocate memory too.
Moreover if encryption runs in async mode, it can block when encryption queue
is full (waiting for condition triggerred from async callback) -> again,
possible deadlock if in common thread.

Of course it will work most of the time, but the design must be robust even
for not common situations.

Milan