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Date:   Wed, 17 May 2023 12:35:46 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     Hugh Dickins <hughd@google.com>
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Subject: Re: [PATCH 15/23] s390: allow pte_offset_map_lock() to fail
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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);

[...]