Re: [PATCH mm-unstable v1 16/20] mm/frame-vector: remove FOLL_FORCE usage

David Hildenbrand
2022-11-27T10:37:13Z

On 16.11.22 11:26, David Hildenbrand wrote:
> FOLL_FORCE is really only for ptrace access. According to commit
> 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are always
> writable"), get_vaddr_frames() currently pins all pages writable as a
> workaround for issues with read-only buffers.
>
> FOLL_FORCE, however, seems to be a legacy leftover as it predates
> commit 707947247e95 ("media: videobuf2-vmalloc: get_userptr: buffers are
> always writable"). Let's just remove it.
>
> Once the read-only buffer issue has been resolved, FOLL_WRITE could
> again be set depending on the DMA direction.
>
> Cc: Hans Verkuil <hverkuil@xs4all.nl>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Tomasz Figa <tfiga@chromium.org>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> drivers/media/common/videobuf2/frame_vector.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
> index 542dde9d2609..062e98148c53 100644
> --- a/drivers/media/common/videobuf2/frame_vector.c
> +++ b/drivers/media/common/videobuf2/frame_vector.c
> @@ -50,7 +50,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
> start = untagged_addr(start);
>
> ret = pin_user_pages_fast(start, nr_frames,
> - FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
> + FOLL_WRITE | FOLL_LONGTERM,
> (struct page **)(vec->ptrs));
> if (ret > 0) {
> vec->got_ref = true;

Hi Andrew,

see the discussion at [1] regarding a conflict and how to proceed with
upstreaming. The conflict would be easy to resolve, however, also
the patch description doesn't make sense anymore with [1].

On top of mm-unstable, reverting this patch and applying [1] gives me
an updated patch:

From 1e66c25f1467c1f1e5f275312f2c6df29308d4df Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 16 Nov 2022 11:26:55 +0100
Subject: [PATCH] mm/frame-vector: remove FOLL_FORCE usage

GUP now supports reliable R/O long-term pinning in COW mappings, such
that we break COW early. MAP_SHARED VMAs only use the shared zeropage so
far in one corner case (DAXFS file with holes), which can be ignored
because GUP does not support long-term pinning in fsdax (see
check_vma_flags()).

Consequently, FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM is no longer required
for reliable R/O long-term pinning: FOLL_LONGTERM is sufficient. So stop
using FOLL_FORCE, which is really only for ptrace access.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
drivers/media/common/videobuf2/frame_vector.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/media/common/videobuf2/frame_vector.c b/drivers/media/common/videobuf2/frame_vector.c
index aad72640f055..8606fdacf5b8 100644
--- a/drivers/media/common/videobuf2/frame_vector.c
+++ b/drivers/media/common/videobuf2/frame_vector.c
@@ -41,7 +41,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames, bool write,
int ret_pin_user_pages_fast = 0;
int ret = 0;
int err;
- unsigned int gup_flags = FOLL_FORCE | FOLL_LONGTERM;
+ unsigned int gup_flags = FOLL_LONGTERM;

if (nr_frames == 0)
return 0;
--
2.38.1

Please let me know how you want to proceed. Ideally, you'd pick up
[1] and apply this updated patch. Also, please tell me if I should
send this updated patch in a separate mail (e.g., as reply to this mail).

[1] https://lkml.kernel.org/r/71bdd3cf-b044-3f12-df58-7c16d5749587@xs4all.nl

--
Thanks,

David / dhildenb