Messages in this thread | | | Subject | Re: [PATCH 2/2] mm/frame-vec: use FOLL_LONGTERM | From | John Hubbard <> | Date | Fri, 2 Oct 2020 15:39:18 -0700 |
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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@intel.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: John Hubbard <jhubbard@nvidia.com> > Cc: Jérôme Glisse <jglisse@redhat.com> > Cc: Jan Kara <jack@suse.cz> > Cc: Dan Williams <dan.j.williams@intel.com> > Cc: linux-mm@kvack.org > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-samsung-soc@vger.kernel.org > Cc: linux-media@vger.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; >
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