linux-kernel - Re: [PATCH v5 3/3] mm: fix double page fault on arm64 if PTE

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Message-ID: <20190919164206.GE6472@arrakis.emea.arm.com>
Date:   Thu, 19 Sep 2019 17:42:06 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jia He <justin.he@....com>
Cc:     Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Suzuki Poulose <Suzuki.Poulose@....com>,
        Punit Agrawal <punitagrawal@...il.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Alex Van Brunt <avanbrunt@...dia.com>,
        Robin Murphy <robin.murphy@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>
Subject: Re: [PATCH v5 3/3] mm: fix double page fault on arm64 if PTE_AF is
 cleared

On Fri, Sep 20, 2019 at 12:12:04AM +0800, Jia He wrote:
> @@ -2152,7 +2163,29 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo
>  	 */
>  	if (unlikely(!src)) {
>  		void *kaddr = kmap_atomic(dst);
> -		void __user *uaddr = (void __user *)(va & PAGE_MASK);
> +		void __user *uaddr = (void __user *)(addr & PAGE_MASK);
> +		pte_t entry;
> +
> +		/* On architectures with software "accessed" bits, we would
> +		 * take a double page fault, so mark it accessed here.
> +		 */
> +		if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
> +			spin_lock(vmf->ptl);
> +			if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> +				entry = pte_mkyoung(vmf->orig_pte);
> +				if (ptep_set_access_flags(vma, addr,
> +							  vmf->pte, entry, 0))
> +					update_mmu_cache(vma, addr, vmf->pte);
> +			} else {
> +				/* Other thread has already handled the fault
> +				 * and we don't need to do anything. If it's
> +				 * not the case, the fault will be triggered
> +				 * again on the same address.
> +				 */
> +				return -1;
> +			}
> +			spin_unlock(vmf->ptl);

Returning with the spinlock held doesn't normally go very well ;).

-- 
Catalin