On 26/04/2024 05:19, Anshuman Khandual wrote:
> On 4/25/24 22:37, Ryan Roberts wrote:
>> __split_huge_pmd_locked() can be called for a present THP, devmap or
>> (non-present) migration entry. It calls pmdp_invalidate()
>> unconditionally on the pmdp and only determines if it is present or not
>> based on the returned old pmd. This is a problem for the migration entry
>> case because pmd_mkinvalid(), called by pmdp_invalidate() must only be
>> called for a present pmd.
>
> pmdp_invalidate() must be called only for present PMD - is this expected
> by core MM ? Does this cause any problem otherwise ?
I'm saying that only calling pmdp_invalidate() on a pte_present()==true pte is
the only semantic that makes sense. And, yes, it causes a problem if called on a
pte_present()==false pte - that's exactly what I'm describing in this commit log.
To labour the point, this is the logical type hierachy of PTEs (and block-mapped
PMDs) as I see it:
---8<----
pte
|- present
| |- valid
| |- invalid
|
|- not_present
|- none
|- swap_pte
present: All fields must be interpretted the way the HW sees them. e.g.
pte_pfn(), pte_write(), pte_dirty(), pte_young(), pte_mkwrite(),
pte_mkold() can all be legitimately used to query and modify the pte.
valid: The HW may access the pte, interpret the fields and create a TLB entry,
etc.
invalid: The HW will never access the pte or create a TLB entry for it.
not_present: The fields are SW-defined. HW never accesses the PTE.
none: Unused; represents a hole
swap_pte: Contains a swap entry and swap pte bits. The contained swap entry
may 1 of a few different types e.g. actual swap entry, migration
entry, hw poison, etc.
---8<----
We test present vs not_present with pte_present()
We test none vs swap_pte with pte_none()
valid vs invalid is slightly more vague. The core-mm can move a PMD from valid
-> invalid by calling pmd_mkinvalid(). But it can't query the state. And it
can't do this generically for a PTE.
Based on that lot, it makes no sense to me that we should permit calling
pmd_mkinvalid() on a non-present pte. Indeed, we don't permit calling
pte_mkwrite() etc on a non-present pte. And those functions are not defensive;
they don't check that the pte is present before making the change. They just
trust that the core-mm will not call them for non-present ptes.
The alternative approach would be to make pmdp_invalidate() defensive so that it
checks the pmd is present before making any changes. But it doesn't semantically
make sense to invalidate a non-present pmd in the first place so why call it
under these circumstances? There is also a practical problem in that some arches
implement their own pmdp_invalidate() so you would want to make all those
defensive too, which would grow the size of the change.
>
>>
>> On arm64 at least, pmd_mkinvalid() will mark the pmd such that any
>> future call to pmd_present() will return true. And therefore any
>
> IIRC the following semantics needs to be followed as expected by core MM.
>
> -------------------------------------------------------------------------
> | PMD states | pmd_present | pmd_trans_huge |
> -------------------------------------------------------------------------
> | Mapped | Yes | Yes |
> -------------------------------------------------------------------------
> | Splitting | Yes | Yes |
> -------------------------------------------------------------------------
> | Migration/Swap | No | No |
> -------------------------------------------------------------------------
Indeed, the problem, as I see it, is if pmd_mkinvalid() is called on a
"Migration/Swap" pmd, then a future call to pmd_present() will return Yes, which
is clearly wrong. pmd_trans_huge() will also return Yes due to:
static inline int pmd_trans_huge(pmd_t pmd)
{
return pmd_val(pmd) && pmd_present(pmd) && !(pmd_val(pmd) & PMD_TABLE_BIT);
}
At least this happens for arm64. Although Zi suggests other arches look like
they will do this too in the other email.
The reason is that arm64's pmd_mkinvalid() unconditionally sets
PMD_PRESENT_INVALID (bit 59) and clears PMD_SECT_VALID (bit 0) in the pte. So
next time pmd_present() is called it will see PMD_PRESENT_INVALID is set and
return true.
>
>
>> lockless pgtable walker could see the migration entry pmd in this state
>> and start interpretting the fields as if it were present, leading to
>> BadThings (TM). GUP-fast appears to be one such lockless pgtable walker.
>
> Could you please explain how bad things might happen ?
See 2 places where pmdp_get_lockless() is called in gup.c, without the PTL.
These could both return the swap pte for which pmd_mkinvalid() has been called.
In both cases, this would lead to the pmd_present() check eroneously returning
true, eventually causing incorrect interpretation of the pte fields. e.g.:
gup_pmd_range()
pmd_t pmd = pmdp_get_lockless(pmdp);
gup_huge_pmd(pmd, ...)
page = nth_page(pmd_page(orig), (addr & ~PMD_MASK) >> PAGE_SHIFT);
page is guff.
Let me know what you think!
Thanks,
Ryan
>
>> I suspect the same is possible on other architectures.
>>
>> Fix this by only calling pmdp_invalidate() for a present pmd. And for
>> good measure let's add a warning to the generic implementation of
>> pmdp_invalidate(). I've manually reviewed all other
>> pmdp_invalidate[_ad]() call sites and believe all others to be
>> conformant.
>>
>> This is a theoretical bug found during code review. I don't have any
>> test case to trigger it in practice.
>>
>> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path")
>> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
>> ---
>>
>> Applies on top of v6.9-rc5. Passes all the mm selftests on arm64.
>>
>> Thanks,
>> Ryan
>>
>>
>> mm/huge_memory.c | 5 +++--
>> mm/pgtable-generic.c | 2 ++
>> 2 files changed, 5 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 89f58c7603b2..80939ad00718 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2513,12 +2513,12 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>> * for this pmd), then we flush the SMP TLB and finally we write the
>> * non-huge version of the pmd entry with pmd_populate.
>> */
>> - old_pmd = pmdp_invalidate(vma, haddr, pmd);
>>
>> - pmd_migration = is_pmd_migration_entry(old_pmd);
>> + pmd_migration = is_pmd_migration_entry(*pmd);
>> if (unlikely(pmd_migration)) {
>> swp_entry_t entry;
>>
>> + old_pmd = *pmd;
>> entry = pmd_to_swp_entry(old_pmd);
>> page = pfn_swap_entry_to_page(entry);
>> write = is_writable_migration_entry(entry);
>> @@ -2529,6 +2529,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>> soft_dirty = pmd_swp_soft_dirty(old_pmd);
>> uffd_wp = pmd_swp_uffd_wp(old_pmd);
>> } else {
>> + old_pmd = pmdp_invalidate(vma, haddr, pmd);
>> page = pmd_page(old_pmd);
>> folio = page_folio(page);
>> if (pmd_dirty(old_pmd)) {
>> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
>> index 4fcd959dcc4d..74e34ea90656 100644
>> --- a/mm/pgtable-generic.c
>> +++ b/mm/pgtable-generic.c
>> @@ -198,6 +198,7 @@ pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp)
>> pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>> pmd_t *pmdp)
>> {
>> + VM_WARN_ON(!pmd_present(*pmdp));
>> pmd_t old = pmdp_establish(vma, address, pmdp, pmd_mkinvalid(*pmdp));
>> flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>> return old;
>> @@ -208,6 +209,7 @@ pmd_t pmdp_invalidate(struct vm_area_struct *vma, unsigned long address,
>> pmd_t pmdp_invalidate_ad(struct vm_area_struct *vma, unsigned long address,
>> pmd_t *pmdp)
>> {
>> + VM_WARN_ON(!pmd_present(*pmdp));
>> return pmdp_invalidate(vma, address, pmdp);
>> }
>> #endif
>> --
>> 2.25.1
>>
>>
