On Mon Jan 5, 2026 at 9:56 AM UTC, David Gow wrote: > On Wed, 24 Dec 2025 at 00:18, Brendan Jackman <[email protected]> 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 <[email protected]> >> --- > > 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).
