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

Petr Vaněk
2025-04-30T13:04:32Z

On Tue, Apr 29, 2025 at 04:02:10PM +0100, Ryan Roberts wrote:
> On 29/04/2025 15:46, David Hildenbrand wrote:
> > On 29.04.25 16:41, Ryan Roberts wrote:
> >> On 29/04/2025 15:29, 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.
> >>>
> >>> Something with XEN is really problematic here.
> >>>
> >>
> >> We are inside a lazy MMU region (arch_enter_lazy_mmu_mode()) at this point,
> >> which I believe XEN uses. If a PTE was written then read back while in lazy mode
> >> you could get a stale value.
> >>
> >> See
> >> https://lore.kernel.org/all/912c7a32-b39c-494f-a29c-4865cd92aeba@agordeev.local/
> >> for an example bug.
> >
> > So if we cannot trust ptep_get() output, then, ... how could we trust anything
> > here and ever possibly batch?
>
> The point is that for a write followed by a read to the same PTE, the read may
> not return what was written. It could return the value of the PTE at the point
> of entry into the lazy mmu mode.
>
> I guess one quick way to test is to hack out lazy mmu support. Something like
> this? (totally untested):

I (blindly) applied the suggested change but I am still seeing the same
issue.

Petr

> ----8<----
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index c4c23190925c..1f0a1a713072 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -541,22 +541,6 @@ static inline void arch_end_context_switch(struct
> task_struct *next)
> PVOP_VCALL1(cpu.end_context_switch, next);
> }
>
> -#define __HAVE_ARCH_ENTER_LAZY_MMU_MODE
> -static inline void arch_enter_lazy_mmu_mode(void)
> -{
> - PVOP_VCALL0(mmu.lazy_mode.enter);
> -}
> -
> -static inline void arch_leave_lazy_mmu_mode(void)
> -{
> - PVOP_VCALL0(mmu.lazy_mode.leave);
> -}
> -
> -static inline void arch_flush_lazy_mmu_mode(void)
> -{
> - PVOP_VCALL0(mmu.lazy_mode.flush);
> -}
> -
> static inline void __set_fixmap(unsigned /* enum fixed_addresses */ idx,
> phys_addr_t phys, pgprot_t flags)
> {
> ----8<----
>