linux-kernel - Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem

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Message-ID: <20190620193619.GK12083@dhcp22.suse.cz>
Date:   Thu, 20 Jun 2019 21:36:19 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Ben Skeggs <bskeggs@...hat.com>, linux-mm@...ck.org,
        nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-nvdimm@...ts.01.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem_free

On Thu 13-06-19 11:43:07, Christoph Hellwig wrote:
> ->mapping isn't even used by HMM users, and the field at the same offset
> in the zone_device part of the union is declared as pad.  (Which btw is
> rather confusing, as DAX uses ->pgmap and ->mapping from two different
> sides of the union, but DAX doesn't use hmm_devmem_free).
> 
> Signed-off-by: Christoph Hellwig <hch@....de>

I cannot really judge here but setting mapping here without any comment
is quite confusing. So if this is safe to remove then it is certainly an
improvement.

> ---
>  mm/hmm.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 0c62426d1257..e1dc98407e7b 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -1347,8 +1347,6 @@ static void hmm_devmem_free(struct page *page, void *data)
>  {
>  	struct hmm_devmem *devmem = data;
>  
> -	page->mapping = NULL;
> -
>  	devmem->ops->free(devmem, page);
>  }
>  
> -- 
> 2.20.1

-- 
Michal Hocko
SUSE Labs