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

[Michal Hocko]

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.
-- 
Michal Hocko
SUSE Labs