linux-kernel - Re: [PATCH -next] arm64/mm: fix a bogus GFP flag in pgd

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Date:   Thu, 6 Jun 2019 00:33:42 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Qian Cai <cai@....pw>, akpm@...ux-foundation.org,
        catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, mhocko@...nel.org,
        linux-mm@...ck.org, vdavydov.dev@...il.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH -next] arm64/mm: fix a bogus GFP flag in pgd_alloc()

On Tue, Jun 04, 2019 at 03:30:20PM +0100, Mark Rutland wrote:
> On Tue, Jun 04, 2019 at 03:23:38PM +0100, Mark Rutland wrote:
> > On Tue, Jun 04, 2019 at 10:00:36AM -0400, Qian Cai wrote:
> > > The commit "arm64: switch to generic version of pte allocation"
> > > introduced endless failures during boot like,
> > > 
> > > kobject_add_internal failed for pgd_cache(285:chronyd.service) (error:
> > > -2 parent: cgroup)
> > > 
> > > It turns out __GFP_ACCOUNT is passed to kernel page table allocations
> > > and then later memcg finds out those don't belong to any cgroup.
> > 
> > Mike, I understood from [1] that this wasn't expected to be a problem,
> > as the accounting should bypass kernel threads.
> > 
> > Was that assumption wrong, or is something different happening here?
> > 
> > > backtrace:
> > >   kobject_add_internal
> > >   kobject_init_and_add
> > >   sysfs_slab_add+0x1a8
> > >   __kmem_cache_create
> > >   create_cache
> > >   memcg_create_kmem_cache
> > >   memcg_kmem_cache_create_func
> > >   process_one_work
> > >   worker_thread
> > >   kthread
> > > 
> > > Signed-off-by: Qian Cai <cai@....pw>
> > > ---
> > >  arch/arm64/mm/pgd.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/mm/pgd.c b/arch/arm64/mm/pgd.c
> > > index 769516cb6677..53c48f5c8765 100644
> > > --- a/arch/arm64/mm/pgd.c
> > > +++ b/arch/arm64/mm/pgd.c
> > > @@ -38,7 +38,7 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> > >  	if (PGD_SIZE == PAGE_SIZE)
> > >  		return (pgd_t *)__get_free_page(gfp);
> > >  	else
> > > -		return kmem_cache_alloc(pgd_cache, gfp);
> > > +		return kmem_cache_alloc(pgd_cache, GFP_PGTABLE_KERNEL);
> > 
> > This is used to allocate PGDs for both user and kernel pagetables (e.g.
> > for the efi runtime services), so while this may fix the regression, I'm
> > not sure it's the right fix.
> 
> I see that since [1], pgd_alloc() was updated to special-case the
> init_mm, which is not sufficient for cases like:
> 
> 	efi_mm.pgd = pgd_alloc(&efi_mm)
> 
> ... which occurs in a kthread.
> 
> So let's have a pgd_alloc_kernel() to make that explicit.

I've hit "send" before seeing this one :)

Well, to be completely on the safe side an explicit pgd_alloc_kernel()
sounds right. Then it won't be subject to future changes in memcg and will
always "Do The Right Thing".
 
> Thanks,
> Mark.
> 

-- 
Sincerely yours,
Mike.