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
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
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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
> ---
> 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
thanks,
--
John Hubbard
NVIDIA
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Re: [PATCH 04/22] mm: don't clear ->mapping in hmm_devmem_free
On Thu, Jun 13, 2019 at 11:43:07AM +0200, 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 > --- > mm/hmm.c | 2 -- > 1 file changed, 2 deletions(-) Hurm, is hmm following this comment from mm_types.h? * If you allocate the page using alloc_pages(), you can use some of the * space in struct page for your own purposes. The five words in the main * union are available, except for bit 0 of the first word which must be * kept clear. Many users use this word to store a pointer to an object * which is guaranteed to be aligned. If you use the same storage as * page->mapping, you must restore it to NULL before freeing the page. Maybe the assumption was that a driver is using ->mapping ? However, nouveau is the only driver that uses this path, and it never touches page->mapping either (nor in -next). It looks like if a driver were to start using mapping then the driver should be responsible to set it back to NULL before being done with the page. Reviewed-by: Jason Gunthorpe Jason ___ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
