Re: [PATCH 1/1] mm: Fix folio_pte_batch() overcount with zero PTEs

David Hildenbrand
2025-04-30T14:37:31Z

On 30.04.25 13:52, Petr Vaněk wrote:
> On Tue, Apr 29, 2025 at 08:56:03PM +0200, David Hildenbrand wrote:
>> On 29.04.25 20:33, Petr Vaněk wrote:
>>> On Tue, Apr 29, 2025 at 05:45:53PM +0200, David Hildenbrand wrote:
>>>> On 29.04.25 16:52, David Hildenbrand wrote:
>>>>> On 29.04.25 16:45, Petr Vaněk wrote:
>>>>>> On Tue, Apr 29, 2025 at 04:29:30PM +0200, David Hildenbrand wrote:
>>>>>>> On 29.04.25 16:22, Petr Vaněk wrote:
>>>>>>>> folio_pte_batch() could overcount the number of contiguous PTEs when
>>>>>>>> pte_advance_pfn() returns a zero-valued PTE and the following PTE in
>>>>>>>> memory also happens to be zero. The loop doesn't break in such a case
>>>>>>>> because pte_same() returns true, and the batch size is advanced by one
>>>>>>>> more than it should be.
>>>>>>>>
>>>>>>>> To fix this, bail out early if a non-present PTE is encountered,
>>>>>>>> preventing the invalid comparison.
>>>>>>>>
>>>>>>>> This issue started to appear after commit 10ebac4f95e7 ("mm/memory:
>>>>>>>> optimize unmap/zap with PTE-mapped THP") and was discovered via git
>>>>>>>> bisect.
>>>>>>>>
>>>>>>>> Fixes: 10ebac4f95e7 ("mm/memory: optimize unmap/zap with PTE-mapped THP")
>>>>>>>> Cc: stable@vger.kernel.org
>>>>>>>> Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
>>>>>>>> ---
>>>>>>>> mm/internal.h | 2 ++
>>>>>>>> 1 file changed, 2 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/mm/internal.h b/mm/internal.h
>>>>>>>> index e9695baa5922..c181fe2bac9d 100644
>>>>>>>> --- a/mm/internal.h
>>>>>>>> +++ b/mm/internal.h
>>>>>>>> @@ -279,6 +279,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
>>>>>>>> dirty = !!pte_dirty(pte);
>>>>>>>> pte = __pte_batch_clear_ignored(pte, flags);
>>>>>>>>
>>>>>>>> + if (!pte_present(pte))
>>>>>>>> + break;
>>>>>>>> if (!pte_same(pte, expected_pte))
>>>>>>>> break;
>>>>>>>
>>>>>>> How could pte_same() suddenly match on a present and non-present PTE.
>>>>>>
>>>>>> In the problematic case pte.pte == 0 and expected_pte.pte == 0 as well.
>>>>>> pte_same() returns a.pte == b.pte -> 0 == 0. Both are non-present PTEs.
>>>>>
>>>>> Observe that folio_pte_batch() was called *with a present pte*.
>>>>>
>>>>> do_zap_pte_range()
>>>>> if (pte_present(ptent))
>>>>> zap_present_ptes()
>>>>> folio_pte_batch()
>>>>>
>>>>> How can we end up with an expected_pte that is !present, if it is based
>>>>> on the provided pte that *is present* and we only used pte_advance_pfn()
>>>>> to advance the pfn?
>>>>
>>>> I've been staring at the code for too long and don't see the issue.
>>>>
>>>> We even have
>>>>
>>>> VM_WARN_ON_FOLIO(!pte_present(pte), folio);
>>>>
>>>> So the initial pteval we got is present.
>>>>
>>>> I don't see how
>>>>
>>>> nr = pte_batch_hint(start_ptep, pte);
>>>> expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
>>>>
>>>> would suddenly result in !pte_present(expected_pte).
>>>
>>> The issue is not happening in __pte_batch_clear_ignored but later in
>>> following line:
>>>
>>> expected_pte = pte_advance_pfn(expected_pte, nr);
>>>
>>> The issue seems to be in __pte function which converts PTE value to
>>> pte_t in pte_advance_pfn, because warnings disappears when I change the
>>> line to
>>>
>>> expected_pte = (pte_t){ .pte = pte_val(expected_pte) + (nr << PFN_PTE_SHIFT) };
>>>
>>> The kernel probably uses __pte function from
>>> arch/x86/include/asm/paravirt.h because it is configured with
>>> CONFIG_PARAVIRT=y:
>>>
>>> static inline pte_t __pte(pteval_t val)
>>> {
>>> return (pte_t) { PVOP_ALT_CALLEE1(pteval_t, mmu.make_pte, val,
>>> "mov %%rdi, %%rax", ALT_NOT_XEN) };
>>> }
>>>
>>> I guess it might cause this weird magic, but I need more time to
>>> understand what it does :)
>
> I understand it slightly more. __pte() uses xen_make_pte(), which calls
> pte_pfn_to_mfn(), however, mfn for this pfn contains INVALID_P2M_ENTRY
> value, therefore the pte_pfn_to_mfn() returns 0, see [1].
>
> I guess that the mfn was invalidated by xen-balloon driver?
>
> [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/xen/mmu_pv.c?h=v6.15-rc4#n408
>
>> What XEN does with basic primitives that convert between pteval and
>> pte_t is beyond horrible.
>>
>> How come set_ptes() that uses pte_next_pfn()->pte_advance_pfn() does not
>> run into this?
>
> I don't know, but I guess it is somehow related to pfn->mfn translation.
>
>> Is it only a problem if we exceed a certain pfn?
>
> No, it is a problem if the corresponding mft to given pfn is invalid.
>
> I am not sure if my original patch is a good fix.

No :)

Maybe it would be
> better to have some sort of native_pte_advance_pfn() which will use
> native_make_pte() rather than __pte(). Or do you think the issue is in
> Xen part?

I think what's happening is that -- under XEN only -- we might get garbage when
calling pte_advance_pfn() and the next PFN would no longer fall into the folio. And
the current code cannot deal with that XEN garbage.

But still not 100% sure.

The following is completely untested, could you give that a try? I
might find some time this evening to test myself and try to further improve it.

From 7d4149a5ea18cba6a694946e59efa9f51d793a4e Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 30 Apr 2025 16:35:12 +0200
Subject: [PATCH] tmp

Signed-off-by: David Hildenbrand <david@redhat.com>
---
mm/internal.h | 29 +++++++++++++----------------
1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index e9695baa59226..a9ea7f62486ec 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -248,11 +248,9 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
bool *any_writable, bool *any_young, bool *any_dirty)
{
- unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
- const pte_t *end_ptep = start_ptep + max_nr;
pte_t expected_pte, *ptep;
bool writable, young, dirty;
- int nr;
+ int nr, cur_nr;

if (any_writable)
*any_writable = false;
@@ -265,11 +263,17 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);

+ /* Limit max_nr to the actual remaining PFNs in the folio. */
+ max_nr = min_t(unsigned long, max_nr,
+ folio_pfn(folio) + folio_nr_pages(folio) - pte_pfn(pte));
+ if (unlikely(max_nr == 1))
+ return 1;
+
nr = pte_batch_hint(start_ptep, pte);
expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
ptep = start_ptep + nr;

- while (ptep < end_ptep) {
+ while (nr < max_nr) {
pte = ptep_get(ptep);
if (any_writable)
writable = !!pte_write(pte);
@@ -282,14 +286,6 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
if (!pte_same(pte, expected_pte))
break;

- /*
- * Stop immediately once we reached the end of the folio. In
- * corner cases the next PFN might fall into a different
- * folio.
- */
- if (pte_pfn(pte) >= folio_end_pfn)
- break;
-
if (any_writable)
*any_writable |= writable;
if (any_young)
@@ -297,12 +293,13 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
if (any_dirty)
*any_dirty |= dirty;

- nr = pte_batch_hint(ptep, pte);
- expected_pte = pte_advance_pfn(expected_pte, nr);
- ptep += nr;
+ cur_nr = pte_batch_hint(ptep, pte);
+ expected_pte = pte_advance_pfn(expected_pte, cur_nr);
+ ptep += cur_nr;
+ nr += cur_nr;
}

- return min(ptep - start_ptep, max_nr);
+ return min(nr, max_nr);
}

/**
--
2.49.0

--
Cheers,

David / dhildenb