Re: [PATCH 2/3] kthread: Add kthread_take_mm()

[Brendan Jackman <jackmanb@google.com>]

On Mon Jan 5, 2026 at 9:56 AM UTC, David Gow wrote:
> On Wed, 24 Dec 2025 at 00:18, Brendan Jackman <jackmanb@google.com> wrote:
>>
>> lib/kunit/user_alloc.c currently uses kthread_use_mm() without a
>> corresponding kthread_unuse_mm(). This is a bug, but fixing it in KUnit
>> makes writing tests that use mms more difficult, because of KUnit's
>> resource/try-catch model.
>>
>> Therefore, introduce a new operation that does what kunit_attach_mm()
>> wants, namely an unbalanced call with cleanup deferred to
>> kthread_exit().
>>
>> This is actually just the same as kthread_use_mm() but without taking a
>> reference on the mm_struct.
>>
>> While adding this, clarify the reference returned by mm_alloc(), since
>> that is what kthread_take_mm() is gonna be paired with, in practice.
>>
>> Signed-off-by: Brendan Jackman <jackmanb@google.com>
>> ---
>
> This makes some sense to me from the KUnit side, though it'd probably
> be nicer to have a way of actually triggering kunit_unuse_mm() at the
> right spot. I'm not sure if we'll want to have tests spawn additional
> threads sharing the same mm in the future, too, which this shouldn't
> make impossible, particularly if we have a requirement that those
> threads don't outlast the original test thread.
>
> Otherwise, Is there a reason we can't mmdrop() from another kthread
> instead of trying to kthread_unuse_mm()? I wouldn't be surprised (it
> doesn't _seem_ right), but seems to work here.

No I think this works and it's actually how I originally wrote the
patch. 

However I think it's very messy, it depends very heavily on the
implementation of kthread_use_mm(), i.e. it is saying "I assume that
everything in kthread_use_mm() gets undone by kthread_exit(), except
that there's exactly one mmdrop() missing". This seems like a natural
conclusion when you've just spent half an hour staring at
kthread.c and drawing up a stupid little ASCII diagram to try and
drill this godforsaken refcount API into your head... But once you step
away from this patchset I think it would look completely bonkers. Here
I'm looking for a way to actually solve this with a proper API.

On the other hand, I'm now adding a weird special kthread API just to
solve this one little problem in KUnit, which people might reasonably
object to.

So yeah I probably should have laid out some other options in the cover
letter. The ones I can obviously see are:

1. The current proposal.

2. Just call mmdrop() from the other kthread and spray comments
   everywhere to try and make it make sense.

3. Find a way to call kthread_unuse_mm() before the kthread dies, with
   some sort of magic in the kunit_try_catch logic.
   But presumably to make that all work with faulting tests etc is gonna
   mean more special APIs, probably worse than kthread_take_mm(). (I did
   not explore this very carefully so it's possible this is easier than
   I guess).