Re: [PATCH v2 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI_P2PDMA

[John Hubbard]

On 6/11/24 11:27 AM, Martin Oliveira wrote:
> This check existed originally due to concerns that P2PDMA needed to copy
> fsdax until pgmap refcounts were fixed (see [1]).
>
> The P2PDMA infrastructure will only call unmap_mapping_range() when the
> underlying device is unbound, and immediately after unmapping it waits
> for the reference of all ZONE_DEVICE pages to be released before
> continuing. This does not allow for a page to be reused and no user
> access fault is therefore possible. It does not have the same problem as
> fsdax.

This sounds great. I'm adding Dan Williams to Cc, in hopes of getting an
ack from him on this point.

> The one minor concern with FOLL_LONGTERM pins is they will block device
> unbind until userspace releases them all.

That seems like a completely reasonable consequence of what you are
doing here, IMHO.

> Co-developed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Signed-off-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Signed-off-by: Martin Oliveira <martin.oliveira@xxxxxxxxxxxxx>
>
> [1]: https://lkml.kernel.org/r/Yy4Ot5MoOhsgYLTQ@xxxxxxxx
> ---
>   mm/gup.c | 5 -----
>   1 file changed, 5 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 00d0a77112f4f..28060e41788d0 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2614,11 +2614,6 @@ static bool is_valid_gup_args(struct page **pages, int *locked,
>   	if (WARN_ON_ONCE((gup_flags & (FOLL_GET | FOLL_PIN)) && !pages))
>   		return false;
>   
> -	/* We want to allow the pgmap to be hot-unplugged at all times */
> -	if (WARN_ON_ONCE((gup_flags & FOLL_LONGTERM) &&
> -			 (gup_flags & FOLL_PCI_P2PDMA)))
> -		return false;
> -

I am not immediately seeing anything wrong with this... :)


>   	*gup_flags_p = gup_flags;
>   	return true;
>   }

thanks,
--
John Hubbard
NVIDIA