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SubjectRe: [PATCH v2] mm: hwpoison: disable memory error handling on 1GB hugepage
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On 02/08/2018 09:30 PM, Punit Agrawal wrote:
> Horiguchi-san,
>
> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> writes:
>
>> Hi Punit,
>>
>> On Mon, Feb 05, 2018 at 03:05:43PM +0000, Punit Agrawal wrote:
>>> Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> writes:
>>>
>
> [...]
>
>>>>
>>>> You can easily reproduce this by calling madvise(MADV_HWPOISON) twice on
>>>> a 1GB hugepage. This happens because get_user_pages_fast() is not aware
>>>> of a migration entry on pud that was created in the 1st madvise() event.
>>>
>>> Maybe I'm doing something wrong but I wasn't able to reproduce the issue
>>> using the test at the end. I get -
>>>
>>> $ sudo ./hugepage
>>>
>>> Poisoning page...once
>>> [ 121.295771] Injecting memory failure for pfn 0x8300000 at process virtual address 0x400000000000
>>> [ 121.386450] Memory failure: 0x8300000: recovery action for huge page: Recovered
>>>
>>> Poisoning page...once again
>>> madvise: Bad address
>>>
>>> What am I missing?
>>
>> The test program below is exactly what I intended, so you did right
>> testing.
>
> Thanks for the confirmation. And the flow outline below.
>
>> I try to guess what could happen. The related code is like below:
>>
>> static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>> int write, struct page **pages, int *nr)
>> {
>> ...
>> do {
>> pud_t pud = READ_ONCE(*pudp);
>>
>> next = pud_addr_end(addr, end);
>> if (pud_none(pud))
>> return 0;
>> if (unlikely(pud_huge(pud))) {
>> if (!gup_huge_pud(pud, pudp, addr, next, write,
>> pages, nr))
>> return 0;
>>
>> pud_none() always returns false for hwpoison entry in any arch.
>> I guess that pud_huge() could behave in undefined manner for hwpoison entry
>> because pud_huge() assumes that a given pud has the present bit set, which
>> is not true for hwpoison entry.
>
> This is where the arm64 helpers behaves differently (though more by
> chance then design). A poisoned pud passes pud_huge() as it doesn't seem
> to be explicitly checking for the present bit.
>
> int pud_huge(pud_t pud)
> {
> return pud_val(pud) && !(pud_val(pud) & PUD_TABLE_BIT);
> }
>
>
> This doesn't lead to a crash as the first thing gup_huge_pud() does is
> check for pud_access_permitted() which does check for the present bit.
>
> I was able to crash the kernel by changing pud_huge() to check for the
> present bit.
>
>> As a result, pud_huge() checks an irrelevant bit used for other
>> purpose depending on non-present page table format of each arch. If
>> pud_huge() returns false for hwpoison entry, we try to go to the lower
>> level and the kernel highly likely to crash. So I guess your kernel
>> fell back the slow path and somehow ended up with returning EFAULT.
>
> Makes sense. Due to the difference above on arm64, it ends up falling
> back to the slow path which eventually returns -EFAULT (via
> follow_hugetlb_page) for poisoned pages.
>
>>
>> So I don't think that the above test result means that errors are properly
>> handled, and the proposed patch should help for arm64.
>
> Although, the deviation of pud_huge() avoids a kernel crash the code
> would be easier to maintain and reason about if arm64 helpers are
> consistent with expectations by core code.
>
> I'll look to update the arm64 helpers once this patch gets merged. But
> it would be helpful if there was a clear expression of semantics for
> pud_huge() for various cases. Is there any version that can be used as
> reference?

Sorry if I misunderstand you, but with this patch there is no non-present
pud entry, so I feel that you don't have to change pud_huge() in arm64.

When we get to have non-present pud entries (by enabling hwpoison or 1GB
hugepage migration), we need to explicitly check pud_present in every page
table walk. So I think the current semantics is like:

if (pud_none(pud))
/* skip this entry */
else if (pud_huge(pud))
/* do something for pud-hugetlb */
else
/* go to next (pmd) level */

and after enabling hwpoison or migartion:

if (pud_none(pud))
/* skip this entry */
else if (!pud_present(pud))
/* do what we need to handle peculiar cases */
else if (pud_huge(pud))
/* do something for pud-hugetlb */
else
/* go to next (pmd) level */

What we did for pmd can also be a reference to what we do for pud.

>
> Also, do you know what the plans are for re-enabling hugepage poisoning
> disabled here?

I'd like to say yes, but it's not specific one because breaking pud isn't
a easy/simple task. But 1GB hugetlb is becoming more important, so we
might have to have code for it.

Thanks,
Naoya Horiguchi

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