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SubjectRe: [PATCH v2] mm: hwpoison: disable memory error handling on 1GB hugepage
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On 02/12/2018 06:48 PM, Michael Ellerman wrote:
> Andrew Morton <akpm@linux-foundation.org> writes:
>
>> On Thu, 08 Feb 2018 12:30:45 +0000 Punit Agrawal <punit.agrawal@arm.com> wrote:
>>
>>>>
>>>> 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?
>>
>> Is that an ack or tested-by?
>>
>> Mike keeps plaintively asking the powerpc developers to take a look,
>> but they remain steadfastly in hiding.
>
> Cc'ing linuxppc-dev is always a good idea :)
>

Thanks Michael,

I was mostly concerned about use cases for soft/hard offline of huge pages
larger than PMD_SIZE on powerpc. I know that powerpc supports PGD_SIZE
huge pages, and soft/hard offline support was specifically added for this.
See, 94310cbcaa3c "mm/madvise: enable (soft|hard) offline of HugeTLB pages
at PGD level"

This patch will disable that functionality. So, at a minimum this is a
'heads up'. If there are actual use cases that depend on this, then more
work/discussions will need to happen. From the e-mail thread on PGD_SIZE
support, I can not tell if there is a real use case or this is just a
'nice to have'.

--
Mike Kravetz

>> Folks, this patch fixes a BUG and is marked for -stable. Can we please
>> prioritize it?
>
> It's not crashing for me (on 4.16-rc1):
>
> # ./huge-poison
> Poisoning page...once
> Poisoning page...once again
> madvise: Bad address
>
> And I guess the above is the expected behaviour?
>
> Looking at the function trace it looks like the 2nd madvise is going
> down reasonable code paths, but I don't know for sure:
>
> 8) | SyS_madvise() {
> 8) | capable() {
> 8) | ns_capable_common() {
> 8) 0.094 us | cap_capable();
> 8) 0.516 us | }
> 8) 1.052 us | }
> 8) | get_user_pages_fast() {
> 8) 0.354 us | gup_pgd_range();
> 8) | get_user_pages_unlocked() {
> 8) 0.050 us | down_read();
> 8) | __get_user_pages() {
> 8) | find_extend_vma() {
> 8) | find_vma() {
> 8) 0.148 us | vmacache_find();
> 8) 0.622 us | }
> 8) 1.064 us | }
> 8) 0.028 us | arch_vma_access_permitted();
> 8) | follow_hugetlb_page() {
> 8) | huge_pte_offset() {
> 8) 0.128 us | __find_linux_pte();
> 8) 0.580 us | }
> 8) 0.048 us | _raw_spin_lock();
> 8) | hugetlb_fault() {
> 8) | huge_pte_offset() {
> 8) 0.034 us | __find_linux_pte();
> 8) 0.434 us | }
> 8) 0.028 us | is_hugetlb_entry_migration();
> 8) 0.032 us | is_hugetlb_entry_hwpoisoned();
> 8) 2.118 us | }
> 8) 4.940 us | }
> 8) 7.468 us | }
> 8) 0.056 us | up_read();
> 8) 8.722 us | }
> 8) + 10.264 us | }
> 8) + 12.212 us | }
>
>
> cheers
>
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