The patch titled Subject: mm: take a page reference when removing device exclusive entries has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-take-a-page-reference-when-removing-device-exclusive-entries.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Alistair Popple apopple@nvidia.com Subject: mm: take a page reference when removing device exclusive entries Date: Thu, 30 Mar 2023 12:25:19 +1100

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 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 ---

mm/memory.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-)

--- a/mm/memory.c~mm-take-a-page-reference-when-removing-device-exclusive-entries +++ a/mm/memory.c @@ -3563,8 +3563,21 @@ static vm_fault_t remove_device_exclusiv struct vm_area_struct *vma = vmf->vma; struct mmu_notifier_range range;

- if (!folio_lock_or_retry(folio, vma->vm_mm, vmf->flags)) + /* + * We need a reference to lock the folio because we don't hold + * the PTL so a racing thread can remove the device-exclusive + * entry and unmap it. If the folio 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 (!folio_try_get(folio)) + return 0; + + if (!folio_lock_or_retry(folio, vma->vm_mm, vmf->flags)) { + folio_put(folio); return VM_FAULT_RETRY; + } mmu_notifier_range_init_owner(&range, MMU_NOTIFY_EXCLUSIVE, 0, vma->vm_mm, vmf->address & PAGE_MASK, (vmf->address & PAGE_MASK) + PAGE_SIZE, NULL); @@ -3577,6 +3590,7 @@ static vm_fault_t remove_device_exclusiv

pte_unmap_unlock(vmf->pte, vmf->ptl); folio_unlock(folio); + folio_put(folio);

mmu_notifier_invalidate_range_end(&range); return 0; _

Patches currently in -mm which might be from apopple@nvidia.com are

mm-take-a-page-reference-when-removing-device-exclusive-entries.patch