Re: [linus:master] [hugetlb] 7118fc2906: kernel_BUG_at_lib/list_debug.c

[Feng Tang]

On Tue, Jan 17, 2023 at 10:25:17AM -0800, Linus Torvalds wrote:
> On Tue, Jan 17, 2023 at 4:22 AM Feng Tang <feng.tang@xxxxxxxxx> wrote:
> >
> > With the following patch to use 'O1' instead 'O2' gcc optoin for
> > page_alloc.c, the list corruption issue can't be reproduced for
> > commit 7118fc2906 in 1000 runs.
> 
> Ugh.
> 
> It would be lovely if you could just narrow it down with
> 
>   #pragma GCC optimize ("O1")
>  ...
>   #pragma GCC optimize ("O2")
> 
> around just that prep_compound_page(), but when I tried it myself I
> get some function attribute mismatch errors.
 
Good to know, thanks! will try this.

> 
> > As is can't be reproduced with X86_64 build, it could be i386
> > compiling related.
> 
> Your particular config causes a huge amount of nasty 64-bit arithmetic
> according to the objdump code, with sequences like
> 
>   c13b3cbb:       83 05 d0 28 6c c5 01    addl   $0x1,0xc56c28d0
>   c13b3cc2:       83 15 d4 28 6c c5 00    adcl   $0x0,0xc56c28d4
> 
> which seems to be just from some coverage profiling being on
> (CONFIG_GCOV?), or something. It makes it very hard to read the code.
> 
> You also have UBSAN enabled, which - again - makes for some really
> grotty asm that hides any actual logic.

Yes, you are right, CONFIG_GCOV_PROFILE_ALL and a bunch of UBSAN
configs are enabled.

AFAIK, this is a random generated config for sanity test, Oliver/Philip
can correct me on this.

> Finally, your objdump version also does some horrendous decoding, like
> 
>   c13b3e29:       8d b4 26 00 00 00 00    lea    0x0(%esi,%eiz,1),%esi
> 
> which is just a 7-byte 'nop' instruction, but again, it makes it
> rather hard to actually read the code.
> 
> With the i386 defconfig, gcc generates a function that is just ~30
> instructions for me, so this makes a huge difference in the legibility
> of the code.
> 
> I wonder if you can recreate the issue with a much more
> straightforward config. By all means, leave DEBUG_PAGEALLOC and SLUB
> debugging on, but without the things like UBSAN and GCOV.

Oliver and I can try this, and will report back.

> > I also objdumped 'prep_compound_page' for vmlinux of 7118fc2906 and
> > its parent commit 48b8d744ea84, which have big difference than the
> > simple 'set_page_count()' change, but I can't tell which part is
> > abnormal, so attach them for further check.
> 
> Yeah, I can't make heads or tails of them either, see above on how
> illegible the objdump files are. And that's despite not even having
> all of prep_compound_page() in them (it's missing
> prep_compound_page.cold, which is probably just UBSAN fixup code, but
> who knows..)
> 
> That said, with the i386 defconfig, the only change from adding
> set_page_count() to the loop seems to be exactly that:
> 
>  .L589:
> -       movl    $1024, 12(%eax)
> +       movl    $0, 28(%eax)
>         addl    $32, %eax
> +       movl    $1024, -20(%eax)
>         movl    %esi, -28(%eax)
>         movl    $0, -12(%eax)
>         cmpl    %edx, %eax
> 
> (don't ask me why gcc does *one* access using the pre-incremented
> pointer, and then the rest to the post-incremented ones, but whatever
> - it means that it's not just "add a mov $0", it's also changing how
> the
> 
>         p->mapping = TAIL_MAPPING;
> 
> instruction is done, which is that
> 
> -       movl    $1024, 12(%eax)
> +       movl    $1024, -20(%eax)
> 
> part of the change)

This version of assembly diff looks more reasonable. One thing I
note is the sizeof(page) with this config is 32, while it's 40 for
the randconfig used in this report.

Thanks for the analyzing and debug suggestion!

- Feng

>              Linus