| Message ID | 20200304142348.48167-1-vincenzo.frascino@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: Make mem_cgroup_id_get_many dependent on MMU and MEMCG_SWAP | expand |
On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote: > mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP > configuration options are enabled. Having them disabled triggers the > following warning at compile time: > > linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined > but not used [-Wunused-function] > static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned > int n) > > Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address > the issue. A similar patch has been proposed recently http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com. The conclusion was that the warning is not really worth adding code. > Cc: Johannes Weiner <hannes@cmpxchg.org> > Cc: Michal Hocko <mhocko@kernel.org> > Cc: Vladimir Davydov <vdavydov.dev@gmail.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com> > --- > mm/memcontrol.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index d09776cd6e10..628cebeb4bdd 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -4794,10 +4794,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg) > } > } > > +#if defined(CONFIG_MEMCG_SWAP) || defined(CONFIG_MMU) > static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n) > { > refcount_add(n, &memcg->id.ref); > } > +#endif > > static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n) > { > -- > 2.25.1
Hi Michal, On 3/4/20 4:53 PM, Michal Hocko wrote: > On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote: >> mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP >> configuration options are enabled. Having them disabled triggers the >> following warning at compile time: >> >> linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined >> but not used [-Wunused-function] >> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned >> int n) >> >> Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address >> the issue. > > A similar patch has been proposed recently > http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com. > The conclusion was that the warning is not really worth adding code. > Thank you for pointing this out, I was not aware of it. I understand that you are against "#ifdeffery" in this case, but isn't it the case of adding at least __maybe_unused? This would prevent people from reporting it over and over again and you to have to push them back :) Let me know what do you think, in case I am happy to change my patch accordingly. [...]
On Thu 05-03-20 09:49:23, Vincenzo Frascino wrote: > Hi Michal, > > On 3/4/20 4:53 PM, Michal Hocko wrote: > > On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote: > >> mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP > >> configuration options are enabled. Having them disabled triggers the > >> following warning at compile time: > >> > >> linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined > >> but not used [-Wunused-function] > >> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned > >> int n) > >> > >> Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address > >> the issue. > > > > A similar patch has been proposed recently > > http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com. > > The conclusion was that the warning is not really worth adding code. > > > > Thank you for pointing this out, I was not aware of it. I understand that you > are against "#ifdeffery" in this case, but isn't it the case of adding at least > __maybe_unused? This would prevent people from reporting it over and over again > and you to have to push them back :) Let me know what do you think, in case I am > happy to change my patch accordingly. We have discussed __maybe_unused in the email thread as well. I am not a great fan of that as mentioed there.
Hi Michal, On 3/5/20 10:00 AM, Michal Hocko wrote: > On Thu 05-03-20 09:49:23, Vincenzo Frascino wrote: >> Hi Michal, >> >> On 3/4/20 4:53 PM, Michal Hocko wrote: >>> On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote: >>>> mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP >>>> configuration options are enabled. Having them disabled triggers the >>>> following warning at compile time: >>>> >>>> linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined >>>> but not used [-Wunused-function] >>>> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned >>>> int n) >>>> >>>> Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address >>>> the issue. >>> >>> A similar patch has been proposed recently >>> http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com. >>> The conclusion was that the warning is not really worth adding code. >>> >> >> Thank you for pointing this out, I was not aware of it. I understand that you >> are against "#ifdeffery" in this case, but isn't it the case of adding at least >> __maybe_unused? This would prevent people from reporting it over and over again >> and you to have to push them back :) Let me know what do you think, in case I am >> happy to change my patch accordingly. > > We have discussed __maybe_unused in the email thread as well. I am not a > great fan of that as mentioed there. > I am aware of this. I was just exploring if there was a possibility of addressing the warning, since if we leave all the warnings in scenarios like randconfig can cause confusion in between real and non real issues. Is there anything we can do?
On Thu 05-03-20 10:46:21, Vincenzo Frascino wrote: [...] > I am aware of this. I was just exploring if there was a possibility of > addressing the warning, since if we leave all the warnings in scenarios like > randconfig can cause confusion in between real and non real issues. I would very much like to address _real_ warnings. This one is just bugus. I do not see a fix that would be sensible. Wrapping the helper by ifdefs is just going to make the resulting code worse because there is nothing really specific to swap there. Marking it __maybe_unused will just paper over the report and allow the function to bitrot when it is not used anymore. So for now I would just live with the warning unless it is really causing any real problems.
On Thu, Mar 05, 2020 at 09:49:23AM +0000, Vincenzo Frascino wrote: > Hi Michal, > > On 3/4/20 4:53 PM, Michal Hocko wrote: > > On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote: > >> mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP > >> configuration options are enabled. Having them disabled triggers the > >> following warning at compile time: > >> > >> linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined > >> but not used [-Wunused-function] > >> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned > >> int n) > >> > >> Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address > >> the issue. > > > > A similar patch has been proposed recently > > http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com. > > The conclusion was that the warning is not really worth adding code. > > > > Thank you for pointing this out, I was not aware of it. I understand that you > are against "#ifdeffery" in this case, but isn't it the case of adding at least > __maybe_unused? This would prevent people from reporting it over and over again > and you to have to push them back :) Let me know what do you think, in case I am > happy to change my patch accordingly. I would ack a patch that adds __maybe_unused. This is a tiny function. If we keep it around a few releases after removing the last user, it costs us absolutely nothing. Eventually somebody will notice and send a patch to remove it. No big deal. There is, however, real cost in keeping bogus warnings around and telling people to ignore them. It's actively lowering the signal-to-noise ratio and normalizing warnings to developers. That's the kind of thing that will actually hide problems in the kernel. We know that the function can be unused in certain scenarios. It's silly to let the compiler continue to warn about it. That's exactly what __maybe_unused is for, so let's use it here.
Hi Johannes, On 3/5/20 4:09 PM, Johannes Weiner wrote: > On Thu, Mar 05, 2020 at 09:49:23AM +0000, Vincenzo Frascino wrote: >> Hi Michal, >> >> On 3/4/20 4:53 PM, Michal Hocko wrote: >>> On Wed 04-03-20 14:23:48, Vincenzo Frascino wrote: >>>> mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP >>>> configuration options are enabled. Having them disabled triggers the >>>> following warning at compile time: >>>> >>>> linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined >>>> but not used [-Wunused-function] >>>> static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned >>>> int n) >>>> >>>> Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address >>>> the issue. >>> >>> A similar patch has been proposed recently >>> http://lkml.kernel.org/r/87fthjh2ib.wl-kuninori.morimoto.gx@renesas.com. >>> The conclusion was that the warning is not really worth adding code. >>> >> >> Thank you for pointing this out, I was not aware of it. I understand that you >> are against "#ifdeffery" in this case, but isn't it the case of adding at least >> __maybe_unused? This would prevent people from reporting it over and over again >> and you to have to push them back :) Let me know what do you think, in case I am >> happy to change my patch accordingly. > > I would ack a patch that adds __maybe_unused. > > This is a tiny function. If we keep it around a few releases after > removing the last user, it costs us absolutely nothing. Eventually > somebody will notice and send a patch to remove it. No big deal. > > There is, however, real cost in keeping bogus warnings around and > telling people to ignore them. It's actively lowering the > signal-to-noise ratio and normalizing warnings to developers. That's > the kind of thing that will actually hide problems in the kernel. > > We know that the function can be unused in certain scenarios. It's > silly to let the compiler continue to warn about it. That's exactly > what __maybe_unused is for, so let's use it here. > I agree with what you are saying. I am going to change my patch accordingly. Thank you.
Johannes Weiner writes: >I would ack a patch that adds __maybe_unused. > >This is a tiny function. If we keep it around a few releases after >removing the last user, it costs us absolutely nothing. Eventually >somebody will notice and send a patch to remove it. No big deal. > >There is, however, real cost in keeping bogus warnings around and >telling people to ignore them. It's actively lowering the >signal-to-noise ratio and normalizing warnings to developers. That's >the kind of thing that will actually hide problems in the kernel. > >We know that the function can be unused in certain scenarios. It's >silly to let the compiler continue to warn about it. That's exactly >what __maybe_unused is for, so let's use it here. Yeah, this is exactly what I was trying to express in the first one[0]. The fact that this patch came around a second time, as expected, just solidifies my concern around the waste to human time. I would also ack a patch that adds __maybe_unused. 0: https://lore.kernel.org/linux-mm/20191217143720.GB131030@chrisdown.name/
diff --git a/mm/memcontrol.c b/mm/memcontrol.c index d09776cd6e10..628cebeb4bdd 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4794,10 +4794,12 @@ static void mem_cgroup_id_remove(struct mem_cgroup *memcg) } } +#if defined(CONFIG_MEMCG_SWAP) || defined(CONFIG_MMU) static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n) { refcount_add(n, &memcg->id.ref); } +#endif static void mem_cgroup_id_put_many(struct mem_cgroup *memcg, unsigned int n) {
mem_cgroup_id_get_many() is currently used only when MMU or MEMCG_SWAP configuration options are enabled. Having them disabled triggers the following warning at compile time: linux/mm/memcontrol.c:4797:13: warning: ‘mem_cgroup_id_get_many’ defined but not used [-Wunused-function] static void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n) Make mem_cgroup_id_get_many() dependent on MMU and MEMCG_SWAP to address the issue. Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@kernel.org> Cc: Vladimir Davydov <vdavydov.dev@gmail.com> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com> --- mm/memcontrol.c | 2 ++ 1 file changed, 2 insertions(+)