Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 21 Aug 2023 06:55:02 +0200 | Subject | Re: [PATCH] mm: remove unintentional voluntary preemption in get_mmap_lock_carefully |
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On Mon, 21 Aug 2023 at 05:58, Matthew Wilcox <willy@infradead.org> wrote: > > The might_sleep() is clearly safe, but I thought of a different take on > the problem you've found, which is that we used to check need_resched > on _every_ page fault, because we used to take the mmap_lock on every > page fault. Now we only check it on the minority of page faults which > can't be handled under the VMA lock. But we can't just slam a > might_resched() into the start of the fault handler, because of the > problem you outlined above.
Bah.
I decided that there is no way the might_sleep() can be the right thing to do inside get_mmap_lock_carefully(), because the whole point of that function existing is that we might have a kernel bug causing a wild pointer access.
And that kernel bug would be about the subsequent oops, not the fact that we might be sleeping in a bad context.
So I have just removed the existing might_sleep() entirely, because both the warning it can generate _and_ the voluntary scheduling point are bad things in that context.
I do think that maybe we should then re-introduce the might_sleep() in some actually appropriate place in the page fault path, which might be 'handle_mm_fault()'.
But I think that's a separate - if related - issue to the whole "this was always the wrong point for might_sleep()" issue that Mateusz noticed.
We are generally much too timid about removing old debug checks that don't really make sense.
Linus
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