On Thu, 8 Jun 2023 12:27:22 -0700 (PDT)
Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> In rare transient cases, not yet made possible, pte_offset_map() and
> pte_offset_map_lock() may not find a page table: handle appropriately.
>
> Add comment on mm's contract with s390 above __zap_zero_pages(),
> and fix old comment there: must be called after THP was disabled.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Acked-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> ---
> arch/s390/kernel/uv.c | 2 ++
> arch/s390/mm/gmap.c | 9 ++++++++-
> arch/s390/mm/pgtable.c | 12 +++++++++---
> 3 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> index cb2ee06df286..3c62d1b218b1 100644
> --- a/arch/s390/kernel/uv.c
> +++ b/arch/s390/kernel/uv.c
> @@ -294,6 +294,8 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>
> rc = -ENXIO;
> ptep = get_locked_pte(gmap->mm, uaddr, &ptelock);
> + if (!ptep)
> + goto out;
> if (pte_present(*ptep) && !(pte_val(*ptep) & _PAGE_INVALID) && pte_write(*ptep)) {
> page = pte_page(*ptep);
> rc = -EAGAIN;
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index dc90d1eb0d55..3a2a31a15ea8 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2537,7 +2537,12 @@ static inline void thp_split_mm(struct mm_struct *mm)
> * Remove all empty zero pages from the mapping for lazy refaulting
> * - This must be called after mm->context.has_pgste is set, to avoid
> * future creation of zero pages
> - * - This must be called after THP was enabled
> + * - This must be called after THP was disabled.
> + *
> + * mm contracts with s390, that even if mm were to remove a page table,
> + * racing with the loop below and so causing pte_offset_map_lock() to fail,
> + * it will never insert a page table containing empty zero pages once
> + * mm_forbids_zeropage(mm) i.e. mm->context.has_pgste is set.
> */
> static int __zap_zero_pages(pmd_t *pmd, unsigned long start,
> unsigned long end, struct mm_walk *walk)
> @@ -2549,6 +2554,8 @@ static int __zap_zero_pages(pmd_t *pmd, unsigned long start,
> spinlock_t *ptl;
>
> ptep = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
> + if (!ptep)
> + break;
> if (is_zero_pfn(pte_pfn(*ptep)))
> ptep_xchg_direct(walk->mm, addr, ptep, __pte(_PAGE_INVALID));
> pte_unmap_unlock(ptep, ptl);
> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> index 6effb24de6d9..3bd2ab2a9a34 100644
> --- a/arch/s390/mm/pgtable.c
> +++ b/arch/s390/mm/pgtable.c
> @@ -829,7 +829,7 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
> default:
> return -EFAULT;
> }
> -
> +again:
> ptl = pmd_lock(mm, pmdp);
> if (!pmd_present(*pmdp)) {
> spin_unlock(ptl);
> @@ -850,6 +850,8 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
> spin_unlock(ptl);
>
> ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> + if (!ptep)
> + goto again;
> new = old = pgste_get_lock(ptep);
> pgste_val(new) &= ~(PGSTE_GR_BIT | PGSTE_GC_BIT |
> PGSTE_ACC_BITS | PGSTE_FP_BIT);
> @@ -938,7 +940,7 @@ int reset_guest_reference_bit(struct mm_struct *mm, unsigned long addr)
> default:
> return -EFAULT;
> }
> -
> +again:
> ptl = pmd_lock(mm, pmdp);
> if (!pmd_present(*pmdp)) {
> spin_unlock(ptl);
> @@ -955,6 +957,8 @@ int reset_guest_reference_bit(struct mm_struct *mm, unsigned long addr)
> spin_unlock(ptl);
>
> ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> + if (!ptep)
> + goto again;
> new = old = pgste_get_lock(ptep);
> /* Reset guest reference bit only */
> pgste_val(new) &= ~PGSTE_GR_BIT;
> @@ -1000,7 +1004,7 @@ int get_guest_storage_key(struct mm_struct *mm, unsigned long addr,
> default:
> return -EFAULT;
> }
> -
> +again:
> ptl = pmd_lock(mm, pmdp);
> if (!pmd_present(*pmdp)) {
> spin_unlock(ptl);
> @@ -1017,6 +1021,8 @@ int get_guest_storage_key(struct mm_struct *mm, unsigned long addr,
> spin_unlock(ptl);
>
> ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
> + if (!ptep)
> + goto again;
> pgste = pgste_get_lock(ptep);
> *key = (pgste_val(pgste) & (PGSTE_ACC_BITS | PGSTE_FP_BIT)) >> 56;
> paddr = pte_val(*ptep) & PAGE_MASK;
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