Re: [PATCH v5] mm/gup: check page hwposion status for coredump.

[David Hildenbrand <david@redhat.com>]

On 31.03.21 06:32, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Wed, Mar 31, 2021 at 10:43:36AM +0800, Aili Yao wrote:
>> On Wed, 31 Mar 2021 01:52:59 +0000 HORIGUCHI NAOYA(堀口　直也) <naoya.horiguchi@nec.com> wrote:
>>> On Fri, Mar 26, 2021 at 03:22:49PM +0100, David Hildenbrand wrote:
>>>> On 26.03.21 15:09, David Hildenbrand wrote:
>>>>> On 22.03.21 12:33, Aili Yao wrote:
>>>>>> When we do coredump for user process signal, this may be one SIGBUS signal
>>>>>> with BUS_MCEERR_AR or BUS_MCEERR_AO code, which means this signal is
>>>>>> resulted from ECC memory fail like SRAR or SRAO, we expect the memory
>>>>>> recovery work is finished correctly, then the get_dump_page() will not
>>>>>> return the error page as its process pte is set invalid by
>>>>>> memory_failure().
>>>>>>
>>>>>> But memory_failure() may fail, and the process's related pte may not be
>>>>>> correctly set invalid, for current code, we will return the poison page,
>>>>>> get it dumped, and then lead to system panic as its in kernel code.
>>>>>>
>>>>>> So check the hwpoison status in get_dump_page(), and if TRUE, return NULL.
>>>>>>
>>>>>> There maybe other scenario that is also better to check hwposion status
>>>>>> and not to panic, so make a wrapper for this check, Thanks to David's
>>>>>> suggestion(<david@redhat.com>).
>>>>>>
>>>>>> Link: https://lkml.kernel.org/r/20210319104437.6f30e80d@alex-virtual-machine
>>>>>> Signed-off-by: Aili Yao <yaoaili@kingsoft.com>
>>>>>> Cc: David Hildenbrand <david@redhat.com>
>>>>>> Cc: Matthew Wilcox <willy@infradead.org>
>>>>>> Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
>>>>>> Cc: Oscar Salvador <osalvador@suse.de>
>>>>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>>>>> Cc: Aili Yao <yaoaili@kingsoft.com>
>>>>>> Cc: stable@vger.kernel.org
>>>>>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>>>>>> ---
>>>>>>     mm/gup.c      |  4 ++++
>>>>>>     mm/internal.h | 20 ++++++++++++++++++++
>>>>>>     2 files changed, 24 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>>>> index e4c224c..6f7e1aa 100644
>>>>>> --- a/mm/gup.c
>>>>>> +++ b/mm/gup.c
>>>>>> @@ -1536,6 +1536,10 @@ struct page *get_dump_page(unsigned long addr)
>>>>>>     				      FOLL_FORCE | FOLL_DUMP | FOLL_GET);
>>>>>>     	if (locked)
>>>>>>     		mmap_read_unlock(mm);
>>>>>
>>>>> Thinking again, wouldn't we get -EFAULT from __get_user_pages_locked()
>>>>> when stumbling over a hwpoisoned page?
>>>>>
>>>>> See __get_user_pages_locked()->__get_user_pages()->faultin_page():
>>>>>
>>>>> handle_mm_fault()->vm_fault_to_errno(), which translates
>>>>> VM_FAULT_HWPOISON to -EFAULT, unless FOLL_HWPOISON is set (-> -EHWPOISON)
>>>>>
>>>>> ?
>>>
>>> We could get -EFAULT, but sometimes not (depends on how memory_failure() fails).
>>>
>>> If we failed to unmap, the page table is not converted to hwpoison entry,
>>> so __get_user_pages_locked() get the hwpoisoned page.
>>>
>>> If we successfully unmapped but failed in truncate_error_page() for example,
>>> the processes mapping the page would get -EFAULT as expected.  But even in
>>> this case, other processes could reach the error page via page cache and
>>> __get_user_pages_locked() for them could return the hwpoisoned page.
>>>
>>>>
>>>> Or doesn't that happen as you describe "But memory_failure() may fail, and
>>>> the process's related pte may not be correctly set invalid" -- but why does
>>>> that happen?
>>>
>>> Simply because memory_failure() doesn't handle some page types like ksm page
>>> and zero page. Or maybe shmem thp also belongs to this class.

Thanks for that info!

>>>
>>>>
>>>> On a similar thought, should get_user_pages() never return a page that has
>>>> HWPoison set? E.g., check also for existing PTEs if the page is hwpoisoned?
>>>
>>> Make sense to me. Maybe inserting hwpoison check into follow_page_pte() and
>>> follow_huge_pmd() would work well.
>>
>> I think we should take more care to broadcast the hwpoison check to other cases,
>> SIGBUS coredump is such a case that it is supposed to not touch the poison page,
>> and if we return NULL for this, the coredump process will get a successful finish.
>>
>> Other cases may also meet the requirements like coredump, but we need to identify it,
>> that's the poison check wrapper's purpose. If not, we may break the integrity of the
>> related action, which may be no better than panic.
> 
> If you worry about regression and would like to make this new behavior conditional,
> we could use FOLL_HWPOISON to specify that the caller is hwpoison-aware so that
> any !FOLL_HWPOISON caller ignores the hwpoison check and works as it does now.
> This approach looks to me helpful because it would encourage developers touching
> gup code to pay attention to FOLL_HWPOISON code.

FOLL_HWPOISON might be the right start, indeed.

-- 
Thanks,

David / dhildenb