Re: [PATCH -next] arm64/mm: fix a bogus GFP flag in pgd_alloc()

Mark Rutland
2019-06-11T13:02:33Z

On Tue, Jun 11, 2019 at 03:41:19PM +0300, Mike Rapoport wrote:
> On Tue, Jun 11, 2019 at 11:03:49AM +0100, Mark Rutland wrote:
> > 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@lca.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.
>
> Sorry for the delay, I'm mostly offline these days.
>
> I wanted to understand first what is the reason for the failure. I've tried
> to reproduce it with qemu, but I failed to find a bootable configuration
> that will have PGD_SIZE != PAGE_SIZE :(

This is the case with 48-bit VA and 64K pages. In that case we have
three levels of table, and the PGD is 1/16th of a page, as it only needs
to resolve 9 bits of virtual address rather than 13.

If you build defconfig + ARM64_64K_PAGES=y, that should be the case:

[mark@lakrids:~/src/linux]% usekorg 8.1.0 aarch64-linux-objdump -d arch/arm64/mm/pgd.o

arch/arm64/mm/pgd.o: file format elf64-littleaarch64

Disassembly of section .text:

0000000000000000 <pgd_alloc>:
0: a9bf7bfd stp x29, x30, [sp, #-16]!
4: 90000000 adrp x0, 0 <pgd_alloc>
8: 5281b801 mov w1, #0xdc0 // #3520
c: 910003fd mov x29, sp
10: f9400000 ldr x0, [x0]
14: 94000000 bl 0 <kmem_cache_alloc>
18: a8c17bfd ldp x29, x30, [sp], #16
1c: d65f03c0 ret

0000000000000020 <pgd_free>:
20: a9bf7bfd stp x29, x30, [sp, #-16]!
24: 90000000 adrp x0, 0 <pgd_alloc>
28: 910003fd mov x29, sp
2c: f9400000 ldr x0, [x0]
30: 94000000 bl 0 <kmem_cache_free>
34: a8c17bfd ldp x29, x30, [sp], #16
38: d65f03c0 ret

Disassembly of section .init.text:

0000000000000000 <pgd_cache_init>:
0: a9bf7bfd stp x29, x30, [sp, #-16]!
4: 52804002 mov w2, #0x200 // #512
8: d2800004 mov x4, #0x0 // #0
c: 910003fd mov x29, sp
10: 2a0203e1 mov w1, w2
14: 52a00083 mov w3, #0x40000 // #262144
18: 90000000 adrp x0, 0 <pgd_cache_init>
1c: 91000000 add x0, x0, #0x0
20: 94000000 bl 0 <kmem_cache_create>
24: 90000001 adrp x1, 0 <pgd_cache_init>
28: a8c17bfd ldp x29, x30, [sp], #16
2c: f9000020 str x0, [x1]
30: d65f03c0 ret

Thanks,
Mark.