linux-kernel - Re: [PATCH v2 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI

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Message-ID: <ac470f75-c276-4f15-b386-e88194e17f13@nvidia.com>
Date: Fri, 14 Jun 2024 19:45:51 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Martin Oliveira <martin.oliveira@...eticom.com>,
	<linux-rdma@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-mm@...ck.org>
CC: Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, Tejun Heo <tj@...nel.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>, Logan Gunthorpe
	<logang@...tatee.com>, Mike Marciniszyn <mike.marciniszyn@...el.com>, Shiraz
 Saleem <shiraz.saleem@...el.com>, Michael Guralnik <michaelgur@...dia.com>,
	"Artemy Kovalyov" <artemyko@...dia.com>
Subject: Re: [PATCH v2 3/4] mm/gup: allow FOLL_LONGTERM & FOLL_PCI_P2PDMA

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@...tatee.com>
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> Signed-off-by: Martin Oliveira <martin.oliveira@...eticom.com>
> 
> [1]: https://lkml.kernel.org/r/Yy4Ot5MoOhsgYLTQ@ziepe.ca
> ---
>   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