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

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Message-Id: <20201002175303.390363-2-daniel.vetter@ffwll.ch>
Date:   Fri,  2 Oct 2020 19:53:03 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     DRI Development <dri-devel@...ts.freedesktop.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        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: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM

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.

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;
-- 
2.28.0