Re: [PATCH mm-unstable v18 06/14] mm/khugepaged: generalize collapse_huge_page for mTHP collapse

[David Hildenbrand (Arm)]

On 6/1/26 05:28, Lance Yang wrote:
> 
> On Sun, May 31, 2026 at 10:00:17PM +0200, David Hildenbrand (Arm) wrote:
>> On 5/31/26 11:39, Lance Yang wrote:
>>>
>>>
>>> Emm ... is it safe to use map_anon_folio_pte_nopf() here?
>>>
>>> At this point pmdp_collapse_flush() has cleared the PMD from the page
>>> tables. The PTE table we are updating is only reachable through the saved
>>> old PMD value, _pmd, until pmd_populate() below.
>>>
>>> map_anon_folio_pte_nopf() does set_ptes() and then calls
>>> update_mmu_cache_range(). Documentation/core-api/cachetlb.rst describes
>>> that hook as:
>>>
>>> "
>>> 	At the end of every page fault, this routine is invoked to tell
>>> 	the architecture specific code that translations now exists
>>> 	in the software page tables for address space "vma->vm_mm"
>>> 	at virtual address "address" for "nr" consecutive pages.
>>> "
>>>
>>> But that does not seem true here yet, since the PTE table is not
>>> reachable from vma->vm_mm when update_mmu_cache_range() is called.
>>>
>>> Should we avoid calling update_mmu_cache_range() until after the PTE
>>> table is reinstalled with pmd_populate()?
>>
>> I recall that update_mmu_cache* users mostly care about updating folios flags,
>> for the folio derived from the PTE ... or flushing caches for the user address.
>>
>> So intuitively I would say "the architecture code doesn't care that the PMD
>> table will only be visible to HW shortly after". The important thing should be
>> that it will definetly happen, and that nothing else is curently there or can be
>> there?
> 
> Ah, fair point.
> 
> I was mostly worried about arch hooks that walk vma->vm_mm again, rather
> than only using the pte pointer passed in. For example, mips does:

Right, a re-walk would be the real problem.

> 
>   update_mmu_cache_range()
>     -> __update_tlb()
>       -> pgd_offset(vma->vm_mm, address)
>       -> pte_offset_map(...)
> 
> and __update_tlb() has this assumption:
> 
> 		/*
> 		 * update_mmu_cache() is called between pte_offset_map_lock()
> 		 * and pte_unmap_unlock(), so we can assume that ptep is not
> 		 * NULL here: and what should be done below if it were NULL?
> 		 */
> 
> So if khugepaged happens to run with current->active_mm == vma->vm_mm
> here, could __update_tlb() hit the none PMD, get NULL from
> pte_offset_map(), and then dereference it?

Likely yes -- that MIPS code is horrible. And the comment in MIPS code
even spells that out. :(

Do you know about other code like that, or is MIPS the only one doing a
re-walk and crossing fingers?

> 
> Just wanted to raise it since some arch code may still have assumptions
> like this, and the always-enable-mTHP work is getting closer ...

Right. I assume set_pte_at() couldn't trigger something similar (re-walk) in arch code,
because we simply provide the ptep. update_mmu_cache_range() only consumes the pte.

> 
> Probably very very very hard to hit, though :)

Delaying update_mmu_cache_range() is nasty, as we'd have to make sure that
nobody can interfere in the meantime ... and the PMD lock will not be sufficient.

Maybe we could reinstall the page table with the cleared (none) entries while
still holding the PTL?

Thinking out loud:

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 5ba298d420b7..e39b750b1e6f 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1413,13 +1413,17 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s
                map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr);
        } else {
                /*
-                * set_ptes is called in map_anon_folio_pte_nopf with the
-                * pmd_ptl lock still held; this is safe as the PMD is expected
-                * to be none. The pmd entry is then repopulated below.
+                * Re-insert the page table with the cleared entries, but
+                * hold the PTL, such that no one can mess with the re-installed
+                * page table until we updated the temporarily-cleared entries
+                * through map_anon_folio_pte_nopf().
                 */
-               map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false);
-               smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */
+               if (pte_ptl != pmd_ptl)
+                       spin_lock(pte_ptl);
                pmd_populate(mm, pmd, pmd_pgtable(_pmd));
+               map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false);
+               if (pte_ptl != pmd_ptl)
+                       spin_unlock(pte_ptl);
        }
        spin_unlock(pmd_ptl);
 


-- 
Cheers,

David