On Tue, Jan 26, 2021 at 04:05:55PM +0100, Michal Hocko wrote:

On Tue 26-01-21 14:48:38, Matthew Wilcox wrote:

quoted

On Mon, Jan 25, 2021 at 11:38:17PM +0200, Mike Rapoport wrote:

quoted

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/ (local)

As for (ab)using NR_SLAB_UNRECLAIMABLE_B, as it was already discussed here:

https://lore.kernel.org/lkml/20201129172625.GD557259@kernel.org/ (local)

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.

Thanks!