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

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Message-ID: <20190604143020.GD24467@lakrids.cambridge.arm.com>
Date:   Tue, 4 Jun 2019 15:30:20 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Qian Cai <cai@....pw>, rppt@...ux.ibm.com
Cc:     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 03:23:38PM +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.

Thanks,
Mark.