On Fri, 13 Sep 2024 08:34:26 -0700 Jakub Kicinski wrote:
> > The second "asm" above (CONFIG_PPC_KERNEL_PREFIXED is not set). I am
> > guessing by searching for "39" in net/core/page_pool.s
> >
> > This is maybe called from page_pool_unref_netmem()
>
> Thanks! The compiler version helped, I can repro with GCC 14.
>
> It's something special about compound page handling on powerpc64,
> AFAICT. I'm guessing that the assembler is mad that we're doing
> an unaligned read:
>
> 3300 ld 8,39(8) # MEM[(const struct atomic64_t *)_29].counter, t
>
> which does indeed look unaligned to a naked eye. If I replace
> virt_to_head_page() with virt_to_page() on line 867 in net/core/page_pool.c
> I get:
>
> 2982 ld 8,40(10) # MEM[(const struct atomic64_t *)_94].counter, t
>
> and that's what we'd expect. It's reading pp_ref_count which is at
> offset 40 in struct net_iov. I'll try to take a closer look at
> the compound page handling, with powerpc assembly book in hand,
> but perhaps this rings a bell for someone?
Oh, okay, I think I understand now. My lack of MM knowledge showing.
So if it's a compound head we do:
static inline unsigned long _compound_head(const struct page *page)
{
unsigned long head = READ_ONCE(page->compound_head);
if (unlikely(head & 1))
return head - 1;
return (unsigned long)page_fixed_fake_head(page);
}
Presumably page->compound_head stores the pointer to the head page.
I'm guessing the compiler is "smart" and decides "why should I do
ld (page - 1) + 40, when I can do ld page + 39 :|
I think it's a compiler bug...
