linux-kernel - Re: [PATCH] mm: add ZONE_DEVICE statistics to smaps

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Date:   Tue, 15 Nov 2016 19:50:15 +0100
From:   Christoph Hellwig <hch@....de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Christoph Hellwig <hch@....de>,
        Dave Hansen <dave.hansen@...el.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: add ZONE_DEVICE statistics to smaps

Hi Dan,

On Mon, Nov 14, 2016 at 07:14:22PM -0800, Dan Williams wrote:
> Wanted to get your opinion on this given your earlier concerns about
> the VM_DAX flag.
> 
> This instead lets an application know how much of a vma is backed by
> ZONE_DEVICE pages, but does not make any indications about the vma
> having DAX semantics or not.  I.e. it is possible that 'device' and
> 'device_huge' are non-zero *and* vma_is_dax() is false.  So, it is
> purely accounting the composition of the present pages in the vma.
> 
> Another option is to have something like 'shared_thp' just to account
> for file backed huge pages that dax can map.  However if ZONE_DEVICE
> is leaking into other use cases I think it makes sense to have it be a
> first class-citizen with respect to accounting alongside
> 'anonymous_thp'.

This counter sounds fine to me, it's a debug tool and not an obvious
abuse candidate like VM_DAX.  But I'll defer to the VM folks for a real
review.