From: Peter Xu <[email protected]>
Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd
unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().
The WARN_ON_ONCE() asserts that an VMA cannot be registered with
userfaultfd-wp if it contains a !normal page, but it's actually possible.
One example is an anonymous vma, register with uffd-wp, read anything will
install a zero page. Then when zap on it, this should trigger.
What's more, removing that WARN_ON_ONCE may not be enough either, because
we should also not rely on "whether it's a normal page" to decide whether
pte marker is needed. For example, one can register wr-protect over some
DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which
case it can have page==NULL for a devmap but we may want to keep the marker
around.
Cc: Muhammad Usama Anjum <[email protected]>
Cc: David Hildenbrand <[email protected]>
Fixes: 0cf18e839f64 ("mm/memory: handle !page case in zap_present_pte() separately")
Signed-off-by: Peter Xu <[email protected]>
---
mm/memory.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/mm/memory.c b/mm/memory.c
index f2bc6dd15eb8..904f70b99498 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1536,7 +1536,9 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
arch_check_zapped_pte(vma, ptent);
tlb_remove_tlb_entry(tlb, pte, addr);
- VM_WARN_ON_ONCE(userfaultfd_wp(vma));
+ if (userfaultfd_pte_wp(vma, ptent))
+ zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
+ details, ptent);
ksm_might_unmap_zero_page(mm, ptent);
return 1;
}
--
2.44.0
On 13.03.24 22:31, [email protected] wrote:
> From: Peter Xu <[email protected]>
>
> Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd
> unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().
Good that I added the WARN_ON_ONCE() :)
>
> The WARN_ON_ONCE() asserts that an VMA cannot be registered with
> userfaultfd-wp if it contains a !normal page, but it's actually possible.
> One example is an anonymous vma, register with uffd-wp, read anything will
> install a zero page. Then when zap on it, this should trigger.
Are you sure? zap_install_uffd_wp_if_needed() contains right at the start:
/* Zap on anonymous always means dropping everything */
if (vma_is_anonymous(vma))
return;
So if that's the case the unit test triggers, I'm confused.
>
> What's more, removing that WARN_ON_ONCE may not be enough either, because
> we should also not rely on "whether it's a normal page" to decide whether
> pte marker is needed. For example, one can register wr-protect over some
> DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which
> case it can have page==NULL for a devmap but we may want to keep the marker
> around.
I thought uffd-wp was limited to specific backends only. But looks like
that changed with UFFD_FEATURE_WP_ASYNC, I guess?
Change itself looks, good. Not sure about the anon_vma example above.
Thanks!
Acked-by: David Hildenbrand <[email protected]>
>
> Cc: Muhammad Usama Anjum <[email protected]>
> Cc: David Hildenbrand <[email protected]>
> Fixes: 0cf18e839f64 ("mm/memory: handle !page case in zap_present_pte() separately")
> Signed-off-by: Peter Xu <[email protected]>
> ---
> mm/memory.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index f2bc6dd15eb8..904f70b99498 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1536,7 +1536,9 @@ static inline int zap_present_ptes(struct mmu_gather *tlb,
> ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> arch_check_zapped_pte(vma, ptent);
> tlb_remove_tlb_entry(tlb, pte, addr);
> - VM_WARN_ON_ONCE(userfaultfd_wp(vma));
> + if (userfaultfd_pte_wp(vma, ptent))
> + zap_install_uffd_wp_if_needed(vma, addr, pte, 1,
> + details, ptent);
> ksm_might_unmap_zero_page(mm, ptent);
> return 1;
> }
--
Cheers,
David / dhildenb
On Wed, Mar 13, 2024 at 11:03:04PM +0100, David Hildenbrand wrote:
> On 13.03.24 22:31, [email protected] wrote:
> > From: Peter Xu <[email protected]>
> >
> > Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd
> > unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().
>
> Good that I added the WARN_ON_ONCE() :)
To be explict, VM_WARN_ON_ONCE. :) And that's my guess that you didn't hit
it when you posted the series and did the tests, as I know latest distros
like Fedora dropped DEBUG_VM, so maybe you had your base config out of
there (but I normally have it irrelevant of that).
>
> >
> > The WARN_ON_ONCE() asserts that an VMA cannot be registered with
> > userfaultfd-wp if it contains a !normal page, but it's actually possible.
> > One example is an anonymous vma, register with uffd-wp, read anything will
> > install a zero page. Then when zap on it, this should trigger.
>
> Are you sure? zap_install_uffd_wp_if_needed() contains right at the start:
>
> /* Zap on anonymous always means dropping everything */
> if (vma_is_anonymous(vma))
> return;
My example is not exactly how the test failed, but should be a simpler
version of it. To trigger this warning I don't think it requires the zero
page to be wr-protected at all or have any pte marker involved.
UFFDIO_REGISTER should suffice, afaiu (feel free to read the example above
again; there's no mention of ioctl(UFFDIO_WRITEPROTECT)).
>
> So if that's the case the unit test triggers, I'm confused.
>
> >
> > What's more, removing that WARN_ON_ONCE may not be enough either, because
> > we should also not rely on "whether it's a normal page" to decide whether
> > pte marker is needed. For example, one can register wr-protect over some
> > DAX regions to track writes when UFFD_FEATURE_WP_ASYNC enabled, in which
> > case it can have page==NULL for a devmap but we may want to keep the marker
> > around.
>
> I thought uffd-wp was limited to specific backends only. But looks like that
> changed with UFFD_FEATURE_WP_ASYNC, I guess?
Correct. That was also what the new PAGEMAP ioctl relies on.
>
>
> Change itself looks, good. Not sure about the anon_vma example above.
>
> Thanks!
>
> Acked-by: David Hildenbrand <[email protected]>
Thanks,
--
Peter Xu
On 13.03.24 23:03, David Hildenbrand wrote:
> On 13.03.24 22:31, [email protected] wrote:
>> From: Peter Xu <[email protected]>
>>
>> Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd
>> unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().
>
> Good that I added the WARN_ON_ONCE() :)
>
>>
>> The WARN_ON_ONCE() asserts that an VMA cannot be registered with
>> userfaultfd-wp if it contains a !normal page, but it's actually possible.
>> One example is an anonymous vma, register with uffd-wp, read anything will
>> install a zero page. Then when zap on it, this should trigger.
>
> Are you sure? zap_install_uffd_wp_if_needed() contains right at the start:
>
> /* Zap on anonymous always means dropping everything */
> if (vma_is_anonymous(vma))
> return;
>
> So if that's the case the unit test triggers, I'm confused.
Ah, got it. It's not that we have to place a marker, just that it can
happen. Of course it can. All makes sense.
Thanks!
--
Cheers,
David / dhildenb
On 13.03.24 23:26, Peter Xu wrote:
> On Wed, Mar 13, 2024 at 11:03:04PM +0100, David Hildenbrand wrote:
>> On 13.03.24 22:31, [email protected] wrote:
>>> From: Peter Xu <[email protected]>
>>>
>>> Commit 0cf18e839f64 of large folio zap work broke uffd-wp. Now mm's uffd
>>> unit test "wp-unpopulated" will trigger this WARN_ON_ONCE().
>>
>> Good that I added the WARN_ON_ONCE() :)
>
> To be explict, VM_WARN_ON_ONCE. :) And that's my guess that you didn't hit
> it when you posted the series and did the tests, as I know latest distros
> like Fedora dropped DEBUG_VM, so maybe you had your base config out of
> there (but I normally have it irrelevant of that).
It's rather surprising that this went unnoticed for quite a while. I
usually do run with DEBUG_VM, but I have a bunch of different VMs where
I tests stuff, likely it was off when I ran the selftests.
--
Cheers,
David / dhildenb