Re: Machine check recovery broken in v6.9-rc1

Oscar Salvador
2024-04-07T04:52:04Z

On Sun, Apr 07, 2024 at 12:08:30AM +0000, Luck, Tony wrote:
> Oscar.
>
> Both the 6.1 and 6.9-rc2 patches make the BUG (and subsequent issues) go away.

Thanks for the switf test Tony!

> Here's what's happening.
>
> When the machine check occurs there's a scramble from various subsystems
> to report the memory error.
>
> ghes_do_memory_failure() calls memory_failure_queue() which later
> calls memory_failure() from a kernel thread. Side note: this happens TWICE
> for each error. Not sure yet if this is a BIOS issue logging more than once.
> or some Linux issues in acpi/apei/ghes.c code.
>
> uc_decode_notifier() [called from a different kernel thread] also calls
> do_memory_failure()
>
> Finally kill_me_maybe() [called from task_work on return to the application
> when returning from the machine check handler] also calls memory_failure()
>
> do_memory_failure() is somewhat prepared for multiple reports of the same
> error. It uses an atomic test and set operation to mark the page as poisoned.
>
> First called to report the error does all the real work. Late arrivals take a
> shorter path, but may still take some action(s) depending on the "flags"
> passed in:
>
> if (TestSetPageHWPoison(p)) {
> pr_err("%#lx: already hardware poisoned\n", pfn);
> res = -EHWPOISON;
> if (flags & MF_ACTION_REQUIRED)
> res = kill_accessing_process(current, pfn, flags);
> if (flags & MF_COUNT_INCREASED)
> put_page(p);
> goto unlock_mutex;
> }

Thanks for the detailed explanation.

> In this case the last to arrive has MF_ACTION_REQUIRED set, so calls
> kill_accessing_process() ... which is in the stack trace that led to the:
>
> kernel BUG at include/linux/swapops.h:88!
>
> I'm not sure that I fully understand your patch. I guess that it is making sure to
> handle the case that the page has already been marked as poisoned?

Basically what is happening is:

1) We mark the page as HWPoison
2) We see that the page is mapped by someone
3) We try to unmap it, and in the process we create a hwpoison swap entry.
See the following chunk from try_to_unmap_one():

"
if (PageHWPoison(subpage) && (flags & TTU_HWPOISON)) {
pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
if (folio_test_hugetlb(folio)) {
hugetlb_count_sub(folio_nr_pages(folio), mm);
set_huge_pte_at(mm, address, pvmw.pte, pteval,
hsz);
} else {
dec_mm_counter(mm, mm_counter(folio));
set_pte_at(mm, address, pvmw.pte, pteval);
}
...
}
"
4) Now there is a second memory event (maybe the previous one has
already finished, I do not think it matters for the sake of this
problem)
5) The second event sees that the page has already been marked as
HWPoison but since it has MF_ACTION_REQUIRED specified, it
goes to kill_accessing_process() to do what its name says.
6) We walk the page tables of the accessing process to see if it has
the poisoned pfn.
7) check_hwpoisoned_entry()
(which is called from
walk_page_range()->walk_{pgd,p4d,pud,pmd}_range()->ops->pmd_entry())
checks whether any of the ptes is poisoned.
8) Since the previous MCE event unmapped the page, pte_present() == 0,
so we want to get the swap entry, and this is where it falls off the
cliff.
See check_hwpoisoned_entry()

static int check_hwpoisoned_entry(pte_t pte, unsigned long addr, short shift,
unsigned long poisoned_pfn, struct to_kill *tk)
{
unsigned long pfn = 0;

if (pte_present(pte)) {
pfn = pte_pfn(pte);
} else {
swp_entry_t swp = pte_to_swp_entry(pte);

if (is_hwpoison_entry(swp))
pfn = swp_offset_pfn(swp);
}
...
}

is_hwpoison_entry() returns true (remember the make_hwpoison_entry()
call we did?)
But when we try to get the pfn from the swap entry, we stumble upon the
VM_BUG_ON(), because is_pfn_swap_entry() only checks for:
is_migration_entry()
is_device_private_entry()
is_device_exclusive_entry()

but it should also check for is_hwpoison_entry().
Since it does not, is_pfn_swap_entry() returns false in our case,
leading to the VM_BUG_ON.

Note that this should only matter in environments where CONFIG_DEBUG_VM
is set.

I hope I shed some light in here.

> Anyway ... thanks for the quick fix. I hope the above helps write a good
> commit message to get this applied and backported to stable.
>
> Tested-by: Tony Luck <tony.luck@intel.com>

Thanks again Tony, much appreciated.

I will write the patch and most likely send it out either today in the
afternoon or tomorrow early in the
morning.

--
Oscar Salvador
SUSE Labs