linux-kernel - Re: [PATCH] mm: disable numa migration faults for dax vmas

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Message-Id: <20161114162529.1a5b08ff90f6f199c1be8cc9@linux-foundation.org>
Date:   Mon, 14 Nov 2016 16:25:29 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Michal Hocko <mhocko@...e.com>, linux-nvdimm@...1.01.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] mm: disable numa migration faults for dax vmas

On Fri, 11 Nov 2016 20:21:41 -0800 Dan Williams <dan.j.williams@...el.com> wrote:

> Mark dax vmas as not migratable to exclude them from task_numa_work().
> This is especially relevant for device-dax which wants to ensure
> predictable access latency and not incur periodic faults.
>
> ...
>
> @@ -177,6 +178,9 @@ static inline bool vma_migratable(struct vm_area_struct *vma)
>  	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
>  		return false;
>  
> +	if (vma_is_dax(vma))
> +		return false;
> +
>  #ifndef CONFIG_ARCH_ENABLE_HUGEPAGE_MIGRATION
>  	if (vma->vm_flags & VM_HUGETLB)
>  		return false;

I don't think the reader could figure out why this code is here, so...  this?

--- a/include/linux/mempolicy.h~mm-disable-numa-migration-faults-for-dax-vmas-fix
+++ a/include/linux/mempolicy.h
@@ -180,6 +180,10 @@ static inline bool vma_migratable(struct
 	if (vma->vm_flags & (VM_IO | VM_PFNMAP))
 		return false;
 
+	/*
+	 * DAX device mappings require predictable access latency, so avoid
+	 * incurring periodic faults.
+	 */
 	if (vma_is_dax(vma))
 		return false;
 
_