Re: [PATCH 3/3] mm:fix gup_pud_range

[Qiujun Huang]

>On 9/20/19 8:51 AM, Qiujun Huang wrote:
>> __get_user_pages_fast try to walk the page table but the
>> hugepage pte is replace by hwpoison swap entry by mca path.
>
>I expect you mean MCE (machine check exception), rather than mca?
Yeah
>
>> ...
>> [15798.177437] mce: Uncorrected hardware memory error in
>>                               user-access at 224f1761c0
>> [15798.180171] MCE 0x224f176: Killing pal_main:6784 due to
>>                               hardware memory corruption
>> [15798.180176] MCE 0x224f176: Killing qemu-system-x86:167336
>>                               due to hardware memory corruption
>> ...
>> [15798.180206] BUG: unable to handle kernel
>> [15798.180226] paging request at ffff891200003000
>> [15798.180236] IP: [<ffffffff8106edae>] gup_pud_range+
>>                               0x13e/0x1e0
>> ...
>>
>> We need to skip the hwpoison entry in gup_pud_range.
>
>It would be nice if this spelled out a little more clearly what's
>wrong. I think you and Aneesh are saying that the entry is really
>a swap entry, created by the MCE response to a bad page?
do_machine_check->
do_memory_failure->
memory_failure->
hwpoison_user_mappings
will updated PUD level PTE entry as a swap entry.

static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
unsigned long address, void *arg)
{
...
if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
if (PageHuge(page)) {
int nr = 1 << compound_order(page);
hugetlb_count_sub(nr, mm);
set_huge_swap_pte_at(mm, address,
pvmw.pte, pteval,
vma_mmu_pagesize(vma));
} else {
dec_mm_counter(mm, mm_counter(page));
set_pte_at(mm, address, pvmw.pte, pteval);
}
...

and, gup_pud_range will reference the pud entry.

gup_pud_range->gup_pmd_range:
static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
int write, struct page **pages, int *nr)
{
unsigned long next;
pmd_t *pmdp;

pmdp = pmd_offset(&pud, addr);
do {
pmd_t pmd = *pmdp;  <--the pmdp is hwpoison swap entry. ffff891200003000
and results in corruption

...
>
>>
>> Signed-off-by: Qiujun Huang <hqjagain@xxxxxxxxx>
>> ---
>>  mm/gup.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 98f13ab..6157ed9 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -2230,6 +2230,8 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>>               next = pud_addr_end(addr, end);
>>               if (pud_none(pud))
>>                       return 0;
>> +             if (unlikely(!pud_present(pud)))
>> +                     return 0;
>
>If the MCE hwpoison behavior puts in swap entries, then it seems like all
>page table walkers would need to check for p*d_present(), and maybe at all
>levels too, right?
I think so
>
>thanks,



On Sat, Sep 21, 2019 at 3:37 AM John Hubbard <jhubbard@xxxxxxxxxx> wrote:
>
> On 9/20/19 8:51 AM, Qiujun Huang wrote:
> > __get_user_pages_fast try to walk the page table but the
> > hugepage pte is replace by hwpoison swap entry by mca path.
>
> I expect you mean MCE (machine check exception), rather than mca?
>
> > ...
> > [15798.177437] mce: Uncorrected hardware memory error in
> >                               user-access at 224f1761c0
> > [15798.180171] MCE 0x224f176: Killing pal_main:6784 due to
> >                               hardware memory corruption
> > [15798.180176] MCE 0x224f176: Killing qemu-system-x86:167336
> >                               due to hardware memory corruption
> > ...
> > [15798.180206] BUG: unable to handle kernel
> > [15798.180226] paging request at ffff891200003000
> > [15798.180236] IP: [<ffffffff8106edae>] gup_pud_range+
> >                               0x13e/0x1e0
> > ...
> >
> > We need to skip the hwpoison entry in gup_pud_range.
>
> It would be nice if this spelled out a little more clearly what's
> wrong. I think you and Aneesh are saying that the entry is really
> a swap entry, created by the MCE response to a bad page?
>
> >
> > Signed-off-by: Qiujun Huang <hqjagain@xxxxxxxxx>
> > ---
> >  mm/gup.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 98f13ab..6157ed9 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2230,6 +2230,8 @@ static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
> >               next = pud_addr_end(addr, end);
> >               if (pud_none(pud))
> >                       return 0;
> > +             if (unlikely(!pud_present(pud)))
> > +                     return 0;
>
> If the MCE hwpoison behavior puts in swap entries, then it seems like all
> page table walkers would need to check for p*d_present(), and maybe at all
> levels too, right?
>
> thanks,
> --
> John Hubbard
> NVIDIA
>
>
> >               if (unlikely(pud_huge(pud))) {
> >                       if (!gup_huge_pud(pud, pudp, addr, next, flags,
> >                                         pages, nr))
> >