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

[Shakeel Butt]

On Wed, Jan 27, 2021 at 11:59 PM Michal Hocko <mhocko@xxxxxxxx> 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@xxxxxxxxxxxxxx/
> > > > >
> > > > > As for (ab)using NR_SLAB_UNRECLAIMABLE_B, as it was already discussed here:
> > > > >
> > > > > https://lore.kernel.org/lkml/20201129172625.GD557259@xxxxxxxxxx/
> > > > >
> > > > > 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.