linux-kernel - Re: 4.0.0-rc4: panic in free

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Date:	Fri, 20 Mar 2015 14:17:43 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Ahern <david.ahern@...cle.com>
Cc:	linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: 4.0.0-rc4: panic in free_block

On Fri, Mar 20, 2015 at 8:07 AM, David Ahern <david.ahern@...cle.com> wrote:
> Instruction DUMP: 86230003  8730f00d  8728f006 <d658c007> 8600c007 8e0ac008 2ac1c002  c658e030  d458e028

Ok, so it's d658c007 that faults, which is that

        ldx  [ %g3 + %g7 ], %o3

instruction.

Looking at your objdump:

> free_block():
> /opt/dahern/linux.git/kbuild/../mm/slab.c:3265
>   55de64:       10 68 00 47     b  %xcc, 55df80 <free_block+0x158>
>   55de68:       85 30 b0 02     srlx  %g2, 2, %g2
> clear_obj_pfmemalloc():
> /opt/dahern/linux.git/kbuild/../mm/slab.c:224
>   55de6c:       98 0b 3f fe     and  %o4, -2, %o4
>   55de70:       d8 76 40 00     stx  %o4, [ %i1 ]
> virt_to_head_page():
> /opt/dahern/linux.git/kbuild/../include/linux/mm.h:554
>   55de74:       c6 5c 80 00     ldx  [ %l2 ], %g3
>   55de78:       ce 5c 40 00     ldx  [ %l1 ], %g7
>   55de7c:       86 23 00 03     sub  %o4, %g3, %g3
>   55de80:       87 30 f0 0d     srlx  %g3, 0xd, %g3
>   55de84:       87 28 f0 06     sllx  %g3, 6, %g3
> test_bit():
> /opt/dahern/linux.git/kbuild/../include/asm-generic/bitops/non-atomic.h:105
>   55de88:       d6 58 c0 07     ldx  [ %g3 + %g7 ], %o3
> virt_to_head_page():
> /opt/dahern/linux.git/kbuild/../include/linux/mm.h:554
>   55de8c:       86 00 c0 07     add  %g3, %g7, %g3

I think that's the load of "page->flags" which is almost certainly
part of that initial

                page = virt_to_head_page(objp);

at the top of the loop in free_block(). In fact, it's probably from
compound_head_fast() which does

        if (unlikely(PageTail(page)))
                if (likely(__get_page_tail(page)))
                        return;

so I think it's about to test the PG_tail bit.

So it's the virt_to_page(x) that has returned 0006100000000000. For
sparc64, that's

   #define virt_to_page(kaddr)    pfn_to_page(__pa(kaddr)>>PAGE_SHIFT)

Looking at the code generation, I think %g7 (0x0006000000000000) is
VMEMMAP_BASE, and %g3 is "pfn << 6", where the "<< 6" is because a
"struct page" is 64 bytes.

And looking at that

        sub  %o4, %g3, %g3

I think that's "__pa(x)", so I think %o4 is 'x'. That also matches the
"and  %o4, -2, %o4", which would be the clear_obj_pfmemalloc().

And %o4 is 0.

In other words, if I read that sparc asm right (and it is very likely
that I do *not*), then "objp" is NULL, and that's why you crash.

That's odd, because we know that objp cannot be NULL in
kmem_slab_free() (even if we allowed it, like with kfree(),
remove_vma() cannot possibly have a NULL vma, since ti dereferences it
multiple times).

So I must be misreading this completely. Somebody with better sparc
debugging mojo should double-check my logic. How would objp be NULL?

                        Linus
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