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Date:   Thu, 28 Jan 2021 08:58:58 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Roman Gushchin <guro@fb.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
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        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
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        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        James Bottomley <jejb@linux.ibm.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
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Subject: Re: [PATCH v16 08/11] secretmem: add memcg accounting
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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.
-- 
Michal Hocko
SUSE Labs