linux-kernel - Re: [PATCH 15/23] s390: allow pte_offset_map

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Date:   Wed, 17 May 2023 12:35:46 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Michal Simek <monstr@...str.eu>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Helge Deller <deller@....de>,
        John David Anglin <dave.anglin@...l.net>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alexandre Ghiti <alexghiti@...osinc.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 15/23] s390: allow pte_offset_map_lock() to fail

On Tue, 9 May 2023 22:01:16 -0700 (PDT)
Hugh Dickins <hughd@...gle.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@...gle.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);

[...]