Messages in this thread | | | Subject | Re: [PATCH] mm-add-vfree_atomic-fix | From | Andrey Ryabinin <> | Date | Tue, 13 Dec 2016 22:21:59 +0300 |
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On 12/13/2016 09:15 PM, Andy Lutomirski wrote:
>>> >>> But not quite acked by me. What happened to the vfree code that >>> causes vfree_deferred to be called in a preemptable context? That >>> sounds like a bug. >> >> Not sure I understand but the above stack points to a preemptible >> context (copy_process). My stack was different and it looks preemptible as well. >> free_thread_stack calls vfree_atomic unconditionally. So I am not sure >> why do you think this is a bug? >> >>> (This code doesn't exist in Linus' tree. What tree does this apply to.) >> >> Anyway, now that I am looking at Andrew's tree I can see [1] which >> doesn't have this_cpu_ptr. So I am not sure where this this_cpu_ptr came >> from. Maybe the previous version of the patch which has shown up in the >> linux-next and Andrew has picked up [2] in the meantime. /me confused >> >> [1] http://www.ozlabs.org/~akpm/mmotm/broken-out/mm-add-vfree_atomic.patch >> [2] http://lkml.kernel.org/r/1481553981-3856-1-git-send-email-aryabinin@virtuozzo.com > > The underlying issue seems to be that we have this shiny new function > vfree_atomic() which doesn't work in *non-atomic* context and that we
It does work non-atomic context. It's fixed now.
> have "kernel/fork: use vfree_atomic() to free thread stack" that calls > vfree_atomic() from non-atomic context.
From both context actually. Usually task stack is freed from atomic context: http://lkml.kernel.org/r/20161019111541.GQ29358@nuc-i3427.alporthouse.com http://lkml.kernel.org/r/CALCETrVqjejgpQVUdem8RK3uxdEgfOZy4cOJqJQjCLtBDnJfyQ@mail.gmail.com
On rare occasions it can be freed from non-atomic context, e.g. error path in copy_process().
> I'm not sure what the motivation of the latter patch was, but ISTM we > should revert it. TBH I'm not quite sure what the purpose of > splitting vfree() and vfree_atomic() was, but I'm not seeing any > reason that the common case of freeing stacks from non-atomic context > should defer the free instead of just doing it right away. > > Andrey, Johannes, why should task stack freeing use vfree_atomic() in > the first place?
Because vfree() now can sleep and task stack freeing usually done in atomic context.
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