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    SubjectRe: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
    On Wed, Sep 30, 2020 at 1:22 PM Michal Hocko <mhocko@suse.com> wrote:

    > > > > I think documenting is useful.
    > > > >
    > > > > Could it be more explicit in what the issue is? Something like:
    > > > >
    > > > > * Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT
    > > > > systems. Therefore, the current low-level allocator implementation does not
    > > > > support being called from special contexts that are atomic on RT - such as
    > > > > NMI and raw_spin_lock. Due to these constraints and considering calling code
    > > > > usually has no control over the PREEMPT_RT configuration, callers of the
    > > > > allocator should avoid calling the allocator from these cotnexts even in
    > > > > non-RT systems.
    > > >
    > > > I do not mind documenting RT specific behavior but as mentioned in other
    > > > reply, this should likely go via RT tree for now. There is likely more
    > > > to clarify about atomicity for PREEMPT_RT.
    > >
    > > I am sorry, I did not understand what you meant by something missing
    > > in Linus Tree. CONFIG_PREEMPT_RT is there.
    >
    > OK, I was not aware we already CONFIG_PREEMPT_RT in the three. There is
    > still a lot from the RT patchset including sleeping spin locks that make
    > a real difference. Or maybe I haven't checked properly.
    >
    > > Could you clarify? Also the CONFIG_PREEMPT_RT is the only thing
    > > driving this requirement for GFP_ATOMIC right? Or are there non-RT
    > > reasons why GFP_ATOMIC allocation cannot be done from
    > > NMI/raw_spin_lock ?
    >
    > I have already sent a clarification patch [1]. NMI is not supported
    > regardless of the preemption mode. Hope this clarifies it a bit.
    >
    > [1] http://lkml.kernel.org/r/20200929123010.5137-1-mhocko@kernel.org

    That works for me. Thanks,

    - Joel

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