| Message ID | 20200929123010.5137-1-mhocko@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: clarify usage of GFP_ATOMIC in !preemptible contexts | expand |
On 29.09.20 14:30, 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). Interesting Reviewed-by: David Hildenbrand <david@redhat.com>
On Tue, Sep 29 2020 at 14:30, Michal Hocko wrote: > 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. Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
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>
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.