Re: [PATCH 15/23] s390: allow pte_offset_map_lock() to fail

Claudio Imbrenda
2023-05-17T10:40:43Z

On Tue, 9 May 2023 22:01:16 -0700 (PDT)
Hugh Dickins <hughd@google.com> 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.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> arch/s390/kernel/uv.c | 2 ++
> arch/s390/mm/gmap.c | 2 ++
> arch/s390/mm/pgtable.c | 12 +++++++++---
> 3 files changed, 13 insertions(+), 3 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..d198fc9475a2 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -2549,6 +2549,8 @@ 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;

so if pte_offset_map_lock fails, we abort and skip both the failed
entry and the rest of the entries?

can pte_offset_map_lock be retried immediately if it fails? (consider
that we currently don't allow THP with KVM guests)

Would something like this:

do {
ptep = pte_offset_map_lock(...);
mb(); /* maybe? */
} while (!ptep);

make sense?

otherwise maybe it's better to return an error and retry the whole
walk_page_range() in s390_enable_sie() ? it's a slow path anyway.

> if (is_zero_pfn(pte_pfn(*ptep)))
> ptep_xchg_direct(walk->mm, addr, ptep, __pte(_PAGE_INVALID));
> pte_unmap_unlock(ptep, ptl); …