Re: [PATCH] mm/khugepaged: free unused page table when setting a huge PMD fails

[Kiryl Shutsemau]

On Thu, Nov 05, 2026 at 01:02:08PM +0000, Kiryl Shutsemau wrote:
> On Thu, Nov 05, 2026 at 01:16:05PM +0100, David Hildenbrand (Arm) wrote:
> > do_set_pmd() can allocate vmf.prealloc_pte on architectures that need a
> > deposited page table, such as PowerPC with the hash MMU. If the PMD is
> > no longer empty when it takes the PMD lock, it returns VM_FAULT_FALLBACK
> > without consuming that table.
> > 
> > Since collapse_pte_mapped_thp() uses mmap_read_lock(), a concurrent
> > userfaultfd operation can install a PTE table before taking the folio
> > lock. If that races with set_huge_pmd(), the unused preallocated table
> > is lost when its local vm_fault goes out of scope.
> > 
> > Free vmf.prealloc_pte on failure, as do_fault() already does. Successful
> > PMD installation consumes the table and clears the pointer.
> > 
> > Having to do this cleanup outside of do_set_pmd() is rather nasty, but
> > other callers depend on it already having been allocated. This needs a
> > cleanup at some point.
> 
> I hate this being outside of do_set_pmd().
> 
> What about making do_set_pmd() freeing ->prealloc_pte if it was
> allocated inside? Nothing changes for caller.
 
Or if we want to keep in set_huge_pmd() then set_huge_pmd() has to
allocate it too. At least, it makes some sense.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov