Device exclusive page table entries are used to prevent CPU access to a page whilst it is being accessed from a device. Typically this is used to implement atomic operations when the underlying bus does not support atomic access. When a CPU thread encounters a device exclusive entry it locks the page and restores the original entry after calling mmu notifiers to signal drivers that exclusive access is no longer available.
The device exclusive entry holds a reference to the page making it safe to access the struct page whilst the entry is present. However the fault handling code does not hold the PTL when taking the page lock. This means if there are multiple threads faulting concurrently on the device exclusive entry one will remove the entry whilst others will wait on the page lock without holding a reference.
This can lead to threads locking or waiting on a folio with a zero refcount. Whilst mmap_lock prevents the pages getting freed via munmap() they may still be freed by a migration. This leads to warnings such as PAGE_FLAGS_CHECK_AT_FREE due to the page being locked when the refcount drops to zero.
Fix this by trying to take a reference on the folio before locking it. The code already checks the PTE under the PTL and aborts if the entry is no longer there. It is also possible the folio has been unmapped, freed and re-allocated allowing a reference to be taken on an unrelated folio. This case is also detected by the PTE check and the folio is unlocked without further changes.
Link: https://lkml.kernel.org/r/20230330012519.804116-1-apopple@nvidia.com Fixes: b756a3b5e7ea ("mm: device exclusive memory access") Signed-off-by: Alistair Popple apopple@nvidia.com Reviewed-by: Ralph Campbell rcampbell@nvidia.com Reviewed-by: John Hubbard jhubbard@nvidia.com Acked-by: David Hildenbrand david@redhat.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Christoph Hellwig hch@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org (cherry picked from commit 7c7b962938ddda6a9cd095de557ee5250706ea88) --- mm/memory.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/mm/memory.c b/mm/memory.c index a4d0f744a458..8d71a82462dd 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3462,8 +3462,21 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf) struct vm_area_struct *vma = vmf->vma; struct mmu_notifier_range range;
- if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags)) + /* + * We need a reference to lock the page because we don't hold + * the PTL so a racing thread can remove the device-exclusive + * entry and unmap it. If the page is free the entry must + * have been removed already. If it happens to have already + * been re-allocated after being freed all we do is lock and + * unlock it. + */ + if (!get_page_unless_zero(page)) + return 0; + + if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags)) { + put_page(page); return VM_FAULT_RETRY; + } mmu_notifier_range_init_owner(&range, MMU_NOTIFY_EXCLUSIVE, 0, vma, vma->vm_mm, vmf->address & PAGE_MASK, (vmf->address & PAGE_MASK) + PAGE_SIZE, NULL); @@ -3476,6 +3489,7 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
pte_unmap_unlock(vmf->pte, vmf->ptl); unlock_page(page); + put_page(page);
mmu_notifier_invalidate_range_end(&range); return 0;