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

[Mike Rapoport]

On Mon, Jan 25, 2021 at 05:54:51PM +0100, Michal Hocko wrote:
> On Thu 21-01-21 14:27:20, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@xxxxxxxxxxxxx>
> > 
> > Account memory consumed by secretmem to memcg. The accounting is updated
> > when the memory is actually allocated and freed.
> 
> What does this mean?

That means that the accounting is updated when secretmem does cma_alloc()
and cma_relase().

> What are the lifetime rules?

Hmm, what do you mean by lifetime rules?

> [...]
> 
> > +static int secretmem_account_pages(struct page *page, gfp_t gfp, int order)
> > +{
> > +	int err;
> > +
> > +	err = memcg_kmem_charge_page(page, gfp, order);
> > +	if (err)
> > +		return err;
> > +
> > +	/*
> > +	 * seceremem caches are unreclaimable kernel allocations, so treat
> > +	 * them as unreclaimable slab memory for VM statistics purposes
> > +	 */
> > +	mod_lruvec_page_state(page, NR_SLAB_UNRECLAIMABLE_B,
> > +			      PAGE_SIZE << order);
> 
> A lot of memcg accounted memory is not reclaimable. Why do you abuse
> SLAB counter when this is not a slab owned memory? Why do you use the
> kmem accounting API when __GFP_ACCOUNT should give you the same without
> this details?

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.

-- 
Sincerely yours,
Mike.