[PATCH 3/3] mm:fix gup_pud_range

[PATCH 3/3] mm:fix gup_pud_range

[Qiujun Huang]

__get_user_pages_fast try to walk the page table but the
hugepage pte is replace by hwpoison swap entry by mca path.
...
[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.

Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
---
 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 (unlikely(pud_huge(pud))) {
 			if (!gup_huge_pud(pud, pudp, addr, next, flags,
 					  pages, nr))
-- 
1.8.3.1

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

[Aneesh Kumar K.V]

On 9/20/19 9:21 PM, 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.
> ...


Can you describe this in more details. I guess you are facing the issue 
with respect PUD level PTE entry that got updated by hwpoison as a swap 
entry. Since we don't specifically check for pud_present(), we walk the 
page table with wrong values and that results in corruption?


> [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.
> 
> Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
> ---
>   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;


You should be able to remove that if (pud_none(pud)) check and just keep 
the pud_present() check?

>   		if (unlikely(pud_huge(pud))) {
>   			if (!gup_huge_pud(pud, pudp, addr, next, flags,
>   					  pages, nr))
> 

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

[John Hubbard]

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@gmail.com>
> ---
>  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))
> 

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@gmail.com>
>> ---
>>  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@nvidia.com> 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@gmail.com>
> > ---
> >  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))
> >

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

[Qiujun Huang]

On Sat, Sep 21, 2019 at 9:19 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 9/20/19 5:33 PM, Qiujun Huang wrote:
> >> On 9/20/19 8:51 AM, Qiujun Huang wrote:
> ...
> >> 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));
>
> OK, that helps. Let's add something approximately like this to the
> commit description:
>
> do_machine_check()
>   do_memory_failure()
>     memory_failure()
>       hw_poison_user_mappings()
>         try_to_unmap()
>           pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
>
> ...and now we have a swap entry that indicates that the page entry
> refers to a bad (and poisoned) page of memory, but gup_fast() at this
> level of the page table was ignoring swap entries, and incorrectly
> assuming that "!pxd_none() == valid and present".
>
> And this was not just a poisoned page problem, but a generaly swap entry
> problem. So, any swap entry type (device memory migration, numa migration,
> or just regular swapping) could lead to the same problem.
>
> Fix this by checking for pxd_present(), instead of pxd_none().
>
>
> > ...
> >>
> >>>
> >>> Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
> >>> ---
> >>>  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
> >>
>
> Should those changes be part of this fix, do you think?

Yes, please.Thanks
>
> thanks,
> --
> John Hubbard
> NVIDIA