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

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Message-ID: <b0584ac6-72e3-08d3-6b76-1ac5e5b3bb4f@nvidia.com>
Date:   Thu, 13 Jun 2019 18:46:29 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Ben Skeggs <bskeggs@...hat.com>
CC:     <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 6/13/19 2:43 AM, 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>
> ---
>  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);
>  }
>  
> 

Yes, I think that line was unnecessary. I see from git history that it was
originally being set to NULL from within __put_devmap_managed_page(), and then
in commit 2fa147bdbf672c53386a8f5f2c7fe358004c3ef8, Dan moved it out of there,
and stashed in specifically here. But it appears to have been unnecessary from
the beginning.

Reviewed-by: John Hubbard <jhubbard@...dia.com> 

thanks,
-- 
John Hubbard
NVIDIA