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

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Date:   Mon, 25 Jan 2021 23:38:17 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v16 08/11] secretmem: add memcg accounting

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@...ux.ibm.com>
> > 
> > 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@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.

-- 
Sincerely yours,
Mike.