Re: [PATCH] kni: fix build with Linux 6.3

[David Marchand <david.marchand@redhat.com>]

On Tue, Feb 28, 2023 at 9:45 PM David Marchand
<david.marchand@redhat.com> wrote:
>
> On Tue, Feb 28, 2023 at 6:29 PM Ferruh Yigit <ferruh.yigit@amd.com> wrote:
> >
> > KNI calls `get_user_pages_remote()` API which is using `FOLL_TOUCH`
> > flag, but `FOLL_TOUCH` is no more in public headers since v6.3,
> > causing a build error.
>
> Something looks strange with what kni was doing.
>
> Looking at get_user_pages_remote implementation, I see it internally
> passes FOLL_TOUCH in addition to passed gup_flags.
> And looking at FOLL_TOUCH definition, it seems natural (to me) that
> this flag would be handled internally.
>
> Maybe it changed over time... but then the question is when did
> passing FOLL_TOUCH become unneeded?

Here is some more info.

get_user_pages_remote() was added in kernel commit 1e9877902dc7
("mm/gup: Introduce get_user_pages_remote()").
At this time, it was passing the FOLL_TOUCH flag internally.

+long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
+               unsigned long start, unsigned long nr_pages,
+               int write, int force, struct page **pages,
+               struct vm_area_struct **vmas)
 {
        return __get_user_pages_locked(tsk, mm, start, nr_pages, write, force,
-                                      pages, vmas, NULL, false, FOLL_TOUCH);
+                                      pages, vmas, NULL, false,
+                                      FOLL_TOUCH | FOLL_REMOTE);
+}
+EXPORT_SYMBOL(get_user_pages_remote);

get_user_pages_remote() later gained the ability to pass gup flags in
kernel commit 9beae1ea8930 ("mm: replace get_user_pages_remote()
write/force parameters with gup_flags").
But FOLL_TOUCH was still added internally.

 long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
                unsigned long start, unsigned long nr_pages,
-               int write, int force, struct page **pages,
+               unsigned int gup_flags, struct page **pages,
                struct vm_area_struct **vmas)
 {
-       unsigned int flags = FOLL_TOUCH | FOLL_REMOTE;
-
-       if (write)
-               flags |= FOLL_WRITE;
-       if (force)
-               flags |= FOLL_FORCE;
-
        return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
-                                      NULL, false, flags);
+                                      NULL, false,
+                                      gup_flags | FOLL_TOUCH | FOLL_REMOTE);
 }


There were other changes in this area of the kernel code, but I did
not notice a change in relation with FOLL_TOUCH.

So I think the dpdk commit e73831dc6c26 ("kni: support userspace VA")
uselessly introduced call to this flag and we can remove it.
Adding author and reviewers of this change.


-- 
David Marchand