linux-kernel - Re: [PATCH 2/2] mm/frame-vec: use FOLL

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Message-ID: <cb56763e-4fda-a783-03ae-7f749ec55981@nvidia.com>
Date:   Fri, 2 Oct 2020 15:39:18 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI Development <dri-devel@...ts.freedesktop.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>, <linux-mm@...ck.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-samsung-soc@...r.kernel.org>, <linux-media@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

On 10/2/20 10:53 AM, Daniel Vetter wrote:
> For $reasons I've stumbled over this code and I'm not sure the change
> to the new gup functions in 55a650c35fea ("mm/gup: frame_vector:
> convert get_user_pages() --> pin_user_pages()") was entirely correct.
> 
> This here is used for long term buffers (not just quick I/O) like
> RDMA, and John notes this in his patch. But I thought the rule for
> these is that they need to add FOLL_LONGTERM, which John's patch
> didn't do.

Yep. The earlier gup --> pup conversion patches were intended to not
have any noticeable behavior changes, and FOLL_LONGTERM, with it's
special cases and such, added some risk that I wasn't ready to take
on yet. Also, FOLL_LONGTERM rules are only *recently* getting firmed
up. So there was some doubt at least in my mind, about which sites
should have it.

But now that we're here, I think it's really good that you've brought
this up. It's definitely time to add FOLL_LONGTERM wherever it's missing.

thanks,
-- 
John Hubbard
NVIDIA

> 
> There is already a dax specific check (added in b7f0554a56f2 ("mm:
> fail get_vaddr_frames() for filesystem-dax mappings")), so this seems
> like the prudent thing to do.
> 
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: Jérôme Glisse <jglisse@...hat.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: linux-mm@...ck.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-samsung-soc@...r.kernel.org
> Cc: linux-media@...r.kernel.org
> ---
> Hi all,
> 
> I stumbled over this and figured typing this patch can't hurt. Really
> just to maybe learn a few things about how gup/pup is supposed to be
> used (we have a bit of that in drivers/gpu), this here isn't really
> ralated to anything I'm doing.
> 
> I'm also wondering whether the explicit dax check should be removed,
> since FOLL_LONGTERM should take care of that already.
> -Daniel
> ---
>   mm/frame_vector.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/frame_vector.c b/mm/frame_vector.c
> index 5d34c9047e9c..3507e09cb3ff 100644
> --- a/mm/frame_vector.c
> +++ b/mm/frame_vector.c
> @@ -35,7 +35,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
>   {
>   	struct mm_struct *mm = current->mm;
>   	struct vm_area_struct *vma;
> -	unsigned int gup_flags = FOLL_WRITE | FOLL_FORCE;
> +	unsigned int gup_flags = FOLL_WRITE | FOLL_FORCE | FOLL_LONGTERM;
>   	int ret = 0;
>   	int err;
>   	int locked;
>