linux-kernel - Re: [PATCH v9 06/24] mm: make pte_unmap

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Date:   Tue, 27 Mar 2018 14:18:17 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
cc:     paulmck@...ux.vnet.ibm.com, peterz@...radead.org,
        akpm@...ux-foundation.org, kirill@...temov.name,
        ak@...ux.intel.com, mhocko@...nel.org, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...ux.vnet.ibm.com,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v9 06/24] mm: make pte_unmap_same compatible with SPF

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2f3e98edc94a..b6432a261e63 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1199,6 +1199,7 @@ static inline void clear_page_pfmemalloc(struct page *page)
>  #define VM_FAULT_NEEDDSYNC  0x2000	/* ->fault did not modify page tables
>  					 * and needs fsync() to complete (for
>  					 * synchronous page faults in DAX) */
> +#define VM_FAULT_PTNOTSAME 0x4000	/* Page table entries have changed */
>  
>  #define VM_FAULT_ERROR	(VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
>  			 VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
> diff --git a/mm/memory.c b/mm/memory.c
> index 21b1212a0892..4bc7b0bdcb40 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2309,21 +2309,29 @@ static bool pte_map_lock(struct vm_fault *vmf)
>   * parts, do_swap_page must check under lock before unmapping the pte and
>   * proceeding (but do_wp_page is only called after already making such a check;
>   * and do_anonymous_page can safely check later on).
> + *
> + * pte_unmap_same() returns:
> + *	0			if the PTE are the same
> + *	VM_FAULT_PTNOTSAME	if the PTE are different
> + *	VM_FAULT_RETRY		if the VMA has changed in our back during
> + *				a speculative page fault handling.
>   */
> -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
> -				pte_t *page_table, pte_t orig_pte)
> +static inline int pte_unmap_same(struct vm_fault *vmf)
>  {
> -	int same = 1;
> +	int ret = 0;
> +
>  #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
>  	if (sizeof(pte_t) > sizeof(unsigned long)) {
> -		spinlock_t *ptl = pte_lockptr(mm, pmd);
> -		spin_lock(ptl);
> -		same = pte_same(*page_table, orig_pte);
> -		spin_unlock(ptl);
> +		if (pte_spinlock(vmf)) {
> +			if (!pte_same(*vmf->pte, vmf->orig_pte))
> +				ret = VM_FAULT_PTNOTSAME;
> +			spin_unlock(vmf->ptl);
> +		} else
> +			ret = VM_FAULT_RETRY;
>  	}
>  #endif
> -	pte_unmap(page_table);
> -	return same;
> +	pte_unmap(vmf->pte);
> +	return ret;
>  }
>  
>  static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
> @@ -2913,7 +2921,8 @@ int do_swap_page(struct vm_fault *vmf)
>  	int exclusive = 0;
>  	int ret = 0;

Initialization is now unneeded.

Otherwise:

Acked-by: David Rientjes <rientjes@...gle.com>