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Message-ID: <77f3f5ce-dd12-4ca2-90cd-0f2226fe26ba@redhat.com>
Date: Mon, 26 Aug 2024 17:26:55 +0200
From: David Hildenbrand <david@...hat.com>
To: Qi Zheng <zhengqi.arch@...edance.com>, hughd@...gle.com,
willy@...radead.org, muchun.song@...ux.dev, vbabka@...nel.org,
akpm@...ux-foundation.org, rppt@...nel.org, vishal.moola@...il.com,
peterx@...hat.com, ryan.roberts@....com, christophe.leroy2@...soprasteria.com
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 02/14] arm: adjust_pte() use pte_offset_map_rw_nolock()
On 22.08.24 09:13, Qi Zheng wrote:
> In do_adjust_pte(), we may modify the pte entry. At this time, the write
> lock of mmap_lock is not held, and the pte_same() check is not performed
> after the PTL held. The corresponding pmd entry may have been modified
> concurrently. Therefore, in order to ensure the stability if pmd entry,
> use pte_offset_map_rw_nolock() to replace pte_offset_map_nolock(), and do
> pmd_same() check after holding the PTL.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> ---
> arch/arm/mm/fault-armv.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/mm/fault-armv.c b/arch/arm/mm/fault-armv.c
> index 831793cd6ff94..de6c7d8a2ddfc 100644
> --- a/arch/arm/mm/fault-armv.c
> +++ b/arch/arm/mm/fault-armv.c
> @@ -94,6 +94,7 @@ static int adjust_pte(struct vm_area_struct *vma, unsigned long address,
> pud_t *pud;
> pmd_t *pmd;
> pte_t *pte;
> + pmd_t pmdval;
> int ret;
>
> pgd = pgd_offset(vma->vm_mm, address);
> @@ -112,16 +113,22 @@ static int adjust_pte(struct vm_area_struct *vma, unsigned long address,
> if (pmd_none_or_clear_bad(pmd))
> return 0;
>
> +again:
> /*
> * This is called while another page table is mapped, so we
> * must use the nested version. This also means we need to
> * open-code the spin-locking.
> */
> - pte = pte_offset_map_nolock(vma->vm_mm, pmd, address, &ptl);
> + pte = pte_offset_map_rw_nolock(vma->vm_mm, pmd, address, &pmdval, &ptl);
> if (!pte)
> return 0;
>
> do_pte_lock(ptl);
> + if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
> + do_pte_unlock(ptl);
> + pte_unmap(pte);
> + goto again;
> + }
>
> ret = do_adjust_pte(vma, address, pfn, pte);
>
Looks correct to me, but I wonder why the missing pmd_same check is not
an issue so far ... any experts? THP on __LINUX_ARM_ARCH__ < 6 is not
really used/possible?
Acked-by: David Hildenbrand <david@...hat.com>
--
Cheers,
David / dhildenb
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