Re: Question about hwpoison handling of 1GB hugepage

[Liu Shixin <liushixin2@huawei.com>]

Hello Naoya,

Is there any progress on memory error handling on 1GB hugepage : )

Thanks,
Liu Shixin

On 2022/4/4 7:42, HORIGUCHI NAOYA(堀口　直也) wrote:
> On Thu, Mar 31, 2022 at 06:56:25PM +0800, Liu Shixin wrote:
>> Hi,
>>
>> Recently, I found a problem with hwpoison 1GB hugepage.
>> I created a process and mapped 1GB hugepage. This process will then fork a
>> child process and write/read this 1GB hugepage. Then I inject hwpoison into
>> this 1GB hugepage. The child process triggers the memory failure and is
>> being killed as expected. After this, the parent process will try to fork a
>> new child process and do the same thing. It is killed again and finally it
>> goes into such an infinite loop. I found this was caused by
>> commit 31286a8484a8 ("mm: hwpoison: disable memory error handling on 1GB hugepage")
> Hello Shixin,
>
> It's little unclear to me about what behavior you are expecting and
> what the infinite loop repeats, could you explain little more about them?
> (I briefly tried to reproduce it, but didn't make it...)
>
>> It looks like there is a bug for hwpoison 1GB hugepage so I try to reproduce
>> the bug described. After trying to revert the patch in an earlier version of
>> the kernel, I reproduce the bug described. Then I try to revert the patch in
>> latest version, and find the bug is no longer reproduced.
>>
>> I compare the code paths of 1 GB hugepage and 2 MB hugepage for second madvise(MADV_HWPOISON),
>> and find that the problem is caused because in gup_pud_range(), pud_none() and
>> pud_huge() both return false and then trigger the bug. But in gup_pmd_range(),
>> the pmd_none() is modified to pmd_present() which will make code return directly.
>> The I find that it is commit 15494520b776 ("mm: fix gup_pud_range") which
>> cause latest version not reproduced. I backport commit 15494520b776 in
>> earlier version and find the bug is no longer reproduced either.
> Thank you for the analysis.
> So this patch might make 31286a8484a8 unnecessary, that's a good news.
>
>> So I'd like to consult that is it the time to revert commit 31286a8484a8?
>> Or if we modify pud_huge to be similar with pmd_huge, is it sufficient?
>>
>> I also noticed there is a TODO comment in memory_failure_hugetlb():
>>     - conversion of a pud that maps an error hugetlb into hwpoison
>>       entry properly works, and
>>     - other mm code walking over page table is aware of pud-aligned
>>       hwpoison entries. 
> These are simply minimum requirements, but might not be sufficient.
> We need testing (with removing 31286a8484a8) to make sure that
> there's no issues on some corner cases.
> (I start to extend existing hugetlb-related testcases to 1GB ones.)
>
> Thanks,
> Naoya Horiguchi
>
>> I'm not sure whether the above fix are sufficient, so is there anything else need
>> to analysis that I haven't considered?