Re: [PATCH v16 08/11] secretmem: add memcg accounting

[Shakeel Butt <shakeelb@google.com>]

On Thu, Jan 28, 2021 at 6:22 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Thu 28-01-21 06:05:11, Shakeel Butt wrote:
> > On Wed, Jan 27, 2021 at 11:59 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Wed 27-01-21 10:42:13, Roman Gushchin wrote:
> > > > On Tue, Jan 26, 2021 at 04:05:55PM +0100, Michal Hocko wrote:
> > > > > On Tue 26-01-21 14:48:38, Matthew Wilcox wrote:
> > > > > > On Mon, Jan 25, 2021 at 11:38:17PM +0200, Mike Rapoport wrote:
> > > > > > > I cannot use __GFP_ACCOUNT because cma_alloc() does not use gfp.
> > > > > > > Besides, kmem accounting with __GFP_ACCOUNT does not seem
> > > > > > > to update stats and there was an explicit request for statistics:
> > > > > > >
> > > > > > > https://lore.kernel.org/lkml/CALo0P13aq3GsONnZrksZNU9RtfhMsZXGWhK1n=xYJWQizCd4Zw@mail.gmail.com/
> > > > > > >
> > > > > > > As for (ab)using NR_SLAB_UNRECLAIMABLE_B, as it was already discussed here:
> > > > > > >
> > > > > > > https://lore.kernel.org/lkml/20201129172625.GD557259@kernel.org/
> > > > > > >
> > > > > > > I think that a dedicated stats counter would be too much at the moment and
> > > > > > > NR_SLAB_UNRECLAIMABLE_B is the only explicit stat for unreclaimable memory.
> > > > > >
> > > > > > That's not true -- Mlocked is also unreclaimable.  And doesn't this
> > > > > > feel more like mlocked memory than unreclaimable slab?  It's also
> > > > > > Unevictable, so could be counted there instead.
> > > > >
> > > > > yes, that is indeed true, except the unreclaimable counter is tracking
> > > > > the unevictable LRUs. These pages are not on any LRU and that can cause
> > > > > some confusion. Maybe they shouldn't be so special and they should live
> > > > > on unevistable LRU and get their stats automagically.
> > > > >
> > > > > I definitely do agree that this would be a better fit than NR_SLAB
> > > > > abuse. But considering that this is somehow even more special than mlock
> > > > > then a dedicated counter sounds as even better fit.
> > > >
> > > > I think it depends on how large these areas will be in practice.
> > > > If they will be measured in single or double digits MBs, a separate entry
> > > > is hardly a good choice: because of the batching the displayed value
> > > > will be in the noise range, plus every new vmstat item adds to the
> > > > struct mem_cgroup size.
> > > >
> > > > If it will be measured in GBs, of course, a separate counter is preferred.
> > > > So I'd suggest to go with NR_SLAB (which should have been named NR_KMEM)
> > > > as now and conditionally switch to a separate counter later.
> > >
> > > I really do not think the overall usage matters when it comes to abusing
> > > other counters. Changing this in future will be always tricky and there
> > > always be our favorite "Can this break userspace" question. Yes we dared
> > > to change meaning of some counters but this is not generally possible.
> > > Just have a look how accounting shmem as a page cache has turned out
> > > being much more tricky than many like.
> > >
> > > Really if a separate counter is a big deal, for which I do not see any
> > > big reason, then this should be accounted as unevictable (as suggested
> > > by Matthew) and ideally pages of those mappings should be sitting in the
> > > unevictable LRU as well unless there is a strong reason against.
> > >
> >
> > Why not decide based on the movability of these pages? If movable then
> > unevictable LRU seems like the right way otherwise NR_SLAB.
>
> I really do not follow. If the page is unevictable then why movability
> matters?

My point was if these pages are very much similar to our existing
definition of unevictable LRU pages then it makes more sense to
account for these pages into unevictable stat.

> I also fail to see why NR_SLAB is even considered considering
> this is completely outside of slab proper.
>
> Really what is the point? What are we trying to achieve by stats? Do we
> want to know how much secret memory is used because that is an
> interesting/important information or do we just want to make some
> accounting?
>
> Just think at it from a practical point of view. I want to know how much
> slab memory is used because it can give me an idea whether kernel is
> consuming unexpected amount of memory. Now I have to subtract _some_
> number to get that information. Where do I get that some number?
>
> We have been creative with counters and it tends to kick back much more
> often than it helps.
>
> I really do not want this to turn into an endless bike shed but either
> this should be accounted as a general type of memory (unevictable would
> be a good fit because that is a userspace memory which is not
> reclaimable) or it needs its own counter to tell how much of this
> specific type of memory is used for this purpose.
>

I suggested having a separate counter in the previous version but got
shot down based on the not-yet-clear benefit of a separate stat for
it.

There is also an option to not add new or use existing stat at this
moment. As there will be more clear use-cases and usage of secretmem,
adding a new stat at that time would be much simpler than changing the
definition of existing stats.