On Thu, Apr 24, 2025 at 8:21 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 23.04.25 10:14, Ming Wang wrote:
> >
> >
> > On 4/23/25 15:07, David Hildenbrand wrote:
> >> On 23.04.25 03:03, Ming Wang wrote:
> >>> When reading /proc/pid/smaps for a process that has mapped a hugetlbfs
> >>> file with MAP_PRIVATE, the kernel might crash inside
> >>> pfn_swap_entry_to_page.
> >>> This occurs on LoongArch under specific conditions.
> >>>
> >>> The root cause involves several steps:
> >>> 1. When the hugetlbfs file is mapped (MAP_PRIVATE), the initial PMD
> >>> (or relevant level) entry is often populated by the kernel during
> >>> mmap()
> >>> with a non-present entry pointing to the architecture's
> >>> invalid_pte_table
> >>> On the affected LoongArch system, this address was observed to
> >>> be 0x90000000031e4000.
> >>> 2. The smaps walker (walk_hugetlb_range -> smaps_hugetlb_range) reads
> >>> this entry.
> >>> 3. The generic is_swap_pte() macro checks `!pte_present() && !
> >>> pte_none()`.
> >>> The entry (invalid_pte_table address) is not present. Crucially,
> >>> the generic pte_none() check (`!(pte_val(pte) & ~_PAGE_GLOBAL)`)
> >>> returns false because the invalid_pte_table address is non-zero.
> >>> Therefore, is_swap_pte() incorrectly returns true.
> >>> 4. The code enters the `else if (is_swap_pte(...))` block.
> >>> 5. Inside this block, it checks `is_pfn_swap_entry()`. Due to a bit
> >>> pattern coincidence in the invalid_pte_table address on LoongArch,
> >>> the embedded generic `is_migration_entry()` check happens to return
> >>> true (misinterpreting parts of the address as a migration type).
> >>> 6. This leads to a call to pfn_swap_entry_to_page() with the bogus
> >>> swap entry derived from the invalid table address.
> >>> 7. pfn_swap_entry_to_page() extracts a meaningless PFN, finds an
> >>> unrelated struct page, checks its lock status (unlocked), and hits
> >>> the `BUG_ON(is_migration_entry(entry) && !PageLocked(p))` assertion.
> >>>
> >>> The original code's intent in the `else if` block seems aimed at handling
> >>> potential migration entries, as indicated by the inner
> >>> `is_pfn_swap_entry()`
> >>> check. The issue arises because the outer `is_swap_pte()` check
> >>> incorrectly
> >>> includes the invalid table pointer case on LoongArch.
> >>
> >> This has a big loongarch smell to it.
> >>
> >> If we end up passing !pte_present() && !pte_none(), then loongarch must
> >> be fixed to filter out these weird non-present entries.
> >>
> >> is_swap_pte() must not succeed on something that is not an actual swap pte.
> >>
> >
> > Hi David,
> >
> > Thanks a lot for your feedback and insightful analysis!
> >
> > You're absolutely right, the core issue here stems from how the generic
> > is_swap_pte() macro interacts with the specific value of
> > invalid_pte_table (or the equivalent invalid table entries for PMD) on
> > the LoongArch architecture. I agree that this has a strong LoongArch
> > characteristic.
> >
> > On the affected LoongArch system, the address used for invalid_pte_table
> > (observed as 0x90000000031e4000 in the vmcore) happens to satisfy both
> > !pte_present() and !pte_none() conditions. This is because:
> > 1. It lacks the _PAGE_PRESENT and _PAGE_PROTNONE bits (correct for an
> > invalid entry).
> > 2. The generic pte_none() check (`!(pte_val(pte) & ~_PAGE_GLOBAL)`)
> > returns false, as the address value itself is non-zero and doesn't match
> > the all-zero (except global bit) pattern.
> > This causes is_swap_pte() to incorrectly return true for these
> > non-mapped, initial entries set up during mmap().
> >
> > The reason my proposed patch changes the condition in
> > smaps_hugetlb_range() from is_swap_pte(ptent) to
> > is_hugetlb_entry_migration(pte) is precisely to leverage an
> > **architecture-level filtering mechanism**, as you suggested LoongArch
> > should provide.
> >
> > This works because is_hugetlb_entry_migration() internally calls
> > `huge_pte_none()`. LoongArch **already provides** an
> > architecture-specific override for huge_pte_none() (via
> > `__HAVE_ARCH_HUGE_PTE_NONE`), which is defined as follows in
> > arch/loongarch/include/asm/pgtable.h:
> >
> > ```
> > static inline int huge_pte_none(pte_t pte)
> > {
> > unsigned long val = pte_val(pte) & ~_PAGE_GLOBAL;
> > /* Check for all zeros (except global) OR if it points to
> > invalid_pte_table */
> > return !val || (val == (unsigned long)invalid_pte_table);
> > }
> > ```
>
> There is now an alternative fix on the list, right?
>
> https://lore.kernel.org/loongarch/20250424083037.2226732-1-wangming01@xxxxxxxxxxx/T/#u
Yes, that one is better.
Huacai
>
> --
> Cheers,
>
> David / dhildenb
>
