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

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Message-ID: <20190611100348.GB26409@lakrids.cambridge.arm.com>
Date:   Tue, 11 Jun 2019 11:03:49 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Qian Cai <cai@....pw>, rppt@...ux.ibm.com
Cc:     Will Deacon <will.deacon@....com>, akpm@...ux-foundation.org,
        catalin.marinas@....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 Mon, Jun 10, 2019 at 01:26:15PM -0400, Qian Cai wrote:
> On Mon, 2019-06-10 at 12:43 +0100, Will Deacon 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.
> > > 
> > > Do we need a separate pgd_alloc_kernel()?
> > 
> > So can I take the above for -rc5, or is somebody else working on a different
> > fix to implement pgd_alloc_kernel()?
> 
> The offensive commit "arm64: switch to generic version of pte allocation" is not
> yet in the mainline, but only in the Andrew's tree and linux-next, and I doubt
> Andrew will push this out any time sooner given it is broken.

I'd assumed that Mike would respin these patches to implement and use
pgd_alloc_kernel() (or take gfp flags) and the updated patches would
replace these in akpm's tree.

Mike, could you confirm what your plan is? I'm happy to review/test
updated patches for arm64.

Thanks,
Mark.