On Wed, 10 Mar 2021 18:13:21 -0800 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> put_page does not correctly handle all calling contexts for hugetlb
> pages. This was recently discussed in the threads [1] and [2].
>
> free_huge_page is the routine called for the final put_page of huegtlb
> pages. Since at least the beginning of git history, free_huge_page has
> acquired the hugetlb_lock to move the page to a free list and possibly
> perform other processing. When this code was originally written, the
> hugetlb_lock should have been made irq safe.
>
> For many years, nobody noticed this situation until lockdep code caught
> free_huge_page being called from irq context. By this time, another
> lock (hugetlb subpool) was also taken in the free_huge_page path. In
> addition, hugetlb cgroup code had been added which could hold
> hugetlb_lock for a considerable period of time. Because of this, commit
> c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in non-task
> context") was added to address the issue of free_huge_page being called
> from irq context. That commit hands off free_huge_page processing to a
> workqueue if !in_task.
>
> The !in_task check handles the case of being called from irq context.
> However, it does not take into account the case when called with irqs
> disabled as in [1].
>
> To complicate matters, functionality has been added to hugetlb
> such that free_huge_page may block/sleep in certain situations. The
> hugetlb_lock is of course dropped before potentially blocking.
>
> One way to handle all calling contexts is to have free_huge_page always
> send pages to the workqueue for processing. This idea was briefly
> discussed here [3], but has some undesirable side effects.
>
> Ideally, the hugetlb_lock should have been irq safe from the beginning
> and any code added to the free_huge_page path should have taken this
> into account. However, this has not happened. The code today does have
> the ability to hand off requests to a workqueue. It does this for calls
> from irq context. Changing the check in the code from !in_task to
> in_atomic would handle the situations when called with irqs disabled.
> However, it does not not handle the case when called with a spinlock
> held. This is needed because the code could block/sleep.
>
> Select PREEMPT_COUNT if HUGETLB_PAGE is enabled so that in_atomic can be
> used to detect all atomic contexts where sleeping is not possible.
>
> [1] https://lore.kernel.org/linux-mm/000000000000f1c03b05bc43aadc@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/YEjji9oAwHuZaZEt@xxxxxxxxxxxxxx/
> [3] https://lore.kernel.org/linux-mm/YDzaAWK41K4gD35V@xxxxxxxxxxxxxx/
>
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -235,6 +235,7 @@ config HUGETLBFS
>
> config HUGETLB_PAGE
> def_bool HUGETLBFS
> + select PREEMPT_COUNT
>
Well this is unfortunate. hugetlb is forcing PREEMPT_COUNT because we
screwed things up.
Did we consider changing the networking code to call a new
free_huge_tlb_from_irq()? So the callee doesn't need to guess.
Or something else?
Is anyone looking onto fixing this for real?
