Re: [PATCHv5 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu <kernelfans@gmail.com>]

On Tue, Mar 3, 2020 at 7:51 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 2/28/20 3:32 AM, Pingfan Liu wrote:
> > FOLL_LONGTERM suggests a pin which is going to be given to hardware and
> > can't move. It would truncate CMA permanently and should be excluded.
> >
> > FOLL_LONGTERM has already been checked in the slow path, but not checked in
> > the fast path, which means a possible leak of CMA page to longterm pinned
> > requirement through this crack.
> >
> > Place a check in try_get_compound_head() in the fast path.
> >
> > Some note about the check:
> > Huge page's subpages have the same migrate type due to either
> > allocation from a free_list[] or alloc_contig_range() with param
> > MIGRATE_MOVABLE. So it is enough to check on a single subpage
> > by is_migrate_cma_page(subpage)
> >
> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Ira Weiny <ira.weiny@intel.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Mike Rapoport <rppt@linux.ibm.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> > Cc: Keith Busch <keith.busch@intel.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Shuah Khan <shuah@kernel.org>
> > To: linux-mm@kvack.org
> > Cc: linux-kernel@vger.kernel.org
> > ---
> >  mm/gup.c | 26 +++++++++++++++++++-------
> >  1 file changed, 19 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index cd8075e..f0d6804 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -33,9 +33,21 @@ struct follow_page_context {
> >   * Return the compound head page with ref appropriately incremented,
> >   * or NULL if that failed.
> >   */
> > -static inline struct page *try_get_compound_head(struct page *page, int refs)
> > +static inline struct page *try_get_compound_head(struct page *page, int refs,
> > +     unsigned int flags)
>
>
> ohhh...please please look at the latest gup.c in mmotm, and this one in particular:
>
>     commit 0ea2781c3de4 mm/gup: track FOLL_PIN pages
>
> ...where you'll see that there is a concept of "try_get*" vs. "try_grab*"). This is going
> to be a huge mess if we do it as above, from a code structure point of view.
>
> The "grab" functions take gup flags, the "get" functions do not.
>
> Anyway, as I said in reply to the cover letter, I'm really uncomfortable with this
> being applied to linux.git. So maybe if we see a fix to mmotm, it will be clearer how
> to port that back to linux.git (assuming that you need 5.6 fixed--do you though?)
Sure, I will read your series and figure out the way to rebase my
patches on mmotm at first.

Thanks,
Pingfan