Re: [PATCH] mm: clarify usage of GFP_ATOMIC in !preemptible contexts

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Sep 29, 2020 at 02:30:10PM +0200, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> There is a general understanding that GFP_ATOMIC/GFP_NOWAIT are
> to be used from atomic contexts. E.g. from within a spin lock or from
> the IRQ context. This is correct but there are some atomic contexts
> where the above doesn't hold. One of them would be an NMI context.
> Page allocator has never supported that and the general fear of this
> context didn't let anybody to actually even try to use the allocator
> there. Good, but let's be more specific about that.
> 
> Another such a context, and that is where people seem to be more daring,
> is raw_spin_lock. Mostly because it simply resembles regular spin lock
> which is supported by the allocator and there is not any implementation
> difference with !RT kernels in the first place. Be explicit that such
> a context is not supported by the allocator. The underlying reason is
> that zone->lock would have to become raw_spin_lock as well and that has
> turned out to be a problem for RT
> (http://lkml.kernel.org/r/87mu305c1w.fsf@nanos.tec.linutronix.de).
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  include/linux/gfp.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 67a0774e080b..2e8370cf60c7 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -238,7 +238,9 @@ struct vm_area_struct;
>   * %__GFP_FOO flags as necessary.
>   *
>   * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
> - * watermark is applied to allow access to "atomic reserves"
> + * watermark is applied to allow access to "atomic reserves".
> + * The current implementation doesn't support NMI and few other strict
> + * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT.
>   *
>   * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
>   * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
> -- 
> 2.28.0
> 

Reviewed-by: Uladzislau Rezki <urezki@gmail.com>