[PATCHv6 0/3] fix omission of check on FOLL_LONGTERM in gup fast path

[PATCHv6 0/3] fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu]

v5 -> v6:
  rebase code to latest linux-mm git tree

  improve commit log

  [3/3], rename GUP_FAST_LONGTERM_BENCHMARK as PIN_FAST_LONGTERM_BENCHMARK due
  to LONGTERM should be a special case of pin page.

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: Christoph Hellwig <hch@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org

Pingfan Liu (3):
  mm/gup: rename nr as nr_pinned in get_user_pages_fast()
  mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path
  mm/gup_benchemark: add LONGTERM_BENCHMARK test in gup fast path

 mm/gup.c                                   | 29 +++++++++++++++++++----------
 mm/gup_benchmark.c                         |  7 +++++++
 tools/testing/selftests/vm/gup_benchmark.c |  6 +++++-
 3 files changed, 31 insertions(+), 11 deletions(-)

--
2.7.5

[PATCHv6 1/3] mm/gup: rename nr as nr_pinned in get_user_pages_fast()

[Pingfan Liu]

To better reflect the held state of pages and make code self-explaining,
rename nr as nr_pinned.

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: 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 | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index e8aaa40..9df77b1 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2735,7 +2735,7 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 					struct page **pages)
 {
 	unsigned long addr, len, end;
-	int nr = 0, ret = 0;
+	int nr_pinned = 0, ret = 0;

 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM |
 				       FOLL_FORCE | FOLL_PIN | FOLL_GET)))
@@ -2754,25 +2754,25 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
 		local_irq_disable();
-		gup_pgd_range(addr, end, gup_flags, pages, &nr);
+		gup_pgd_range(addr, end, gup_flags, pages, &nr_pinned);
 		local_irq_enable();
-		ret = nr;
+		ret = nr_pinned;
 	}

-	if (nr < nr_pages) {
+	if (nr_pinned < nr_pages) {
 		/* Try to get the remaining pages with get_user_pages */
-		start += nr << PAGE_SHIFT;
-		pages += nr;
+		start += nr_pinned << PAGE_SHIFT;
+		pages += nr_pinned;

-		ret = __gup_longterm_unlocked(start, nr_pages - nr,
+		ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned,
 					      gup_flags, pages);

 		/* Have to be a bit careful with return values */
-		if (nr > 0) {
+		if (nr_pinned > 0) {
 			if (ret < 0)
-				ret = nr;
+				ret = nr_pinned;
 			else
-				ret += nr;
+				ret += nr_pinned;
 		}
 	}

--
2.7.5

[PATCHv6 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu]

FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
going to be given to hardware and can't move. It would truncate CMA
permanently and should be excluded.

In gup slow path, slow path, where
__gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
but in fast path, there lacks such a check, which means a possible leak of
CMA page to longterm pinned.

Place a check in try_grab_compound_head() in the fast path to fix the leak,
and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
migrate the page.

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: Christoph Hellwig <hch@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 mm/gup.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 9df77b1..78132cf 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -89,6 +89,15 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 		int orig_refs = refs;

 		/*
+		 * 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 subpage.
+		 */
+		if (unlikely(flags & FOLL_LONGTERM) &&
+			is_migrate_cma_page(page))
+			return NULL;
+
+		/*
 		 * When pinning a compound page of order > 1 (which is what
 		 * hpage_pincount_available() checks for), use an exact count to
 		 * track it, via hpage_pincount_add/_sub().
--
2.7.5

[PATCHv6 3/3] mm/gup_benchemark: add LONGTERM_BENCHMARK test in gup fast path

[Pingfan Liu]

Introduce a PIN_FAST_LONGTERM_BENCHMARK ioctl to test longterm pin in gup fast
path.

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: Christoph Hellwig <hch@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Alexander Duyck <alexander.duyck@gmail.com>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
 mm/gup_benchmark.c                         | 7 +++++++
 tools/testing/selftests/vm/gup_benchmark.c | 6 +++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/gup_benchmark.c b/mm/gup_benchmark.c
index be690fa..09fba20 100644
--- a/mm/gup_benchmark.c
+++ b/mm/gup_benchmark.c
@@ -10,6 +10,7 @@
 #define GUP_BENCHMARK		_IOWR('g', 3, struct gup_benchmark)
 #define PIN_FAST_BENCHMARK	_IOWR('g', 4, struct gup_benchmark)
 #define PIN_BENCHMARK		_IOWR('g', 5, struct gup_benchmark)
+#define PIN_FAST_LONGTERM_BENCHMARK	_IOWR('g', 6, struct gup_benchmark)

 struct gup_benchmark {
 	__u64 get_delta_usec;
@@ -101,6 +102,11 @@ static int __gup_benchmark_ioctl(unsigned int cmd,
 			nr = get_user_pages_fast(addr, nr, gup->flags,
 						 pages + i);
 			break;
+		case PIN_FAST_LONGTERM_BENCHMARK:
+			nr = get_user_pages_fast(addr, nr,
+					gup->flags | FOLL_LONGTERM,
+					pages + i);
+			break;
 		case GUP_LONGTERM_BENCHMARK:
 			nr = get_user_pages(addr, nr,
 					    gup->flags | FOLL_LONGTERM,
@@ -166,6 +172,7 @@ static long gup_benchmark_ioctl(struct file *filep, unsigned int cmd,
 	case GUP_BENCHMARK:
 	case PIN_FAST_BENCHMARK:
 	case PIN_BENCHMARK:
+	case PIN_FAST_LONGTERM_BENCHMARK:
 		break;
 	default:
 		return -EINVAL;
diff --git a/tools/testing/selftests/vm/gup_benchmark.c b/tools/testing/selftests/vm/gup_benchmark.c
index 43b4dfe..f024ff3 100644
--- a/tools/testing/selftests/vm/gup_benchmark.c
+++ b/tools/testing/selftests/vm/gup_benchmark.c
@@ -21,6 +21,7 @@
 /* Similar to above, but use FOLL_PIN instead of FOLL_GET. */
 #define PIN_FAST_BENCHMARK	_IOWR('g', 4, struct gup_benchmark)
 #define PIN_BENCHMARK		_IOWR('g', 5, struct gup_benchmark)
+#define PIN_FAST_LONGTERM_BENCHMARK	_IOWR('g', 6, struct gup_benchmark)

 /* Just the flags we need, copied from mm.h: */
 #define FOLL_WRITE	0x01	/* check pte is writable */
@@ -44,7 +45,7 @@ int main(int argc, char **argv)
 	char *file = "/dev/zero";
 	char *p;

-	while ((opt = getopt(argc, argv, "m:r:n:f:abtTLUuwSH")) != -1) {
+	while ((opt = getopt(argc, argv, "m:r:n:f:abtTlLUuwSH")) != -1) {
 		switch (opt) {
 		case 'a':
 			cmd = PIN_FAST_BENCHMARK;
@@ -67,6 +68,9 @@ int main(int argc, char **argv)
 		case 'T':
 			thp = 0;
 			break;
+		case 'l':
+			cmd = PIN_FAST_LONGTERM_BENCHMARK;
+			break;
 		case 'L':
 			cmd = GUP_LONGTERM_BENCHMARK;
 			break;
--
2.7.5

Re: [PATCHv6 1/3] mm/gup: rename nr as nr_pinned in get_user_pages_fast()

[Christoph Hellwig]

On Mon, Mar 16, 2020 at 12:34:02PM +0800, Pingfan Liu wrote:
> To better reflect the held state of pages and make code self-explaining,
> rename nr as nr_pinned.

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCHv6 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Christoph Hellwig]

On Mon, Mar 16, 2020 at 12:34:03PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
> going to be given to hardware and can't move. It would truncate CMA
> permanently and should be excluded.
> 
> In gup slow path, slow path, where
> __gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
> but in fast path, there lacks such a check, which means a possible leak of
> CMA page to longterm pinned.
> 
> Place a check in try_grab_compound_head() in the fast path to fix the leak,
> and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
> migrate the page.
> 
> 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: Christoph Hellwig <hch@infradead.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> To: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  mm/gup.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 9df77b1..78132cf 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -89,6 +89,15 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
>  		int orig_refs = refs;
> 
>  		/*
> +		 * 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 subpage.
> +		 */
> +		if (unlikely(flags & FOLL_LONGTERM) &&
> +			is_migrate_cma_page(page))
> +			return NULL;

Wrong indentation.  We either align two tabs for continuation
statements, or after the opening brace of the previous line.  With a
likely or unlikely thrown in the former IMHO looks much better.  E.g.

		if (unlikely(flags & FOLL_LONGTERM) &&
				is_migrate_cma_page(page))
			return NULL;

Re: [PATCHv6 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu]

On Mon, Mar 16, 2020 at 4:55 PM Christoph Hellwig <hch@infradead.org> wrote:
>
[...]
>
> Wrong indentation.  We either align two tabs for continuation
> statements, or after the opening brace of the previous line.  With a
> likely or unlikely thrown in the former IMHO looks much better.  E.g.
>
>                 if (unlikely(flags & FOLL_LONGTERM) &&
>                                 is_migrate_cma_page(page))
>                         return NULL;
>
OK, thanks. I will send out V7 to fix it.

Regards,
Pingfan

[PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu]

FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
going to be given to hardware and can't move. It would truncate CMA
permanently and should be excluded.

In gup slow path, slow path, where
__gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
but in fast path, there lacks such a check, which means a possible leak of
CMA page to longterm pinned.

Place a check in try_grab_compound_head() in the fast path to fix the leak,
and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
migrate the page.

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: Christoph Hellwig <hch@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
To: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
---
v6 -> v7: fix coding style issue
 mm/gup.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index 9df77b1..0a536d7 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -89,6 +89,15 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 		int orig_refs = refs;

 		/*
+		 * 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 subpage.
+		 */
+		if (unlikely(flags & FOLL_LONGTERM) &&
+				is_migrate_cma_page(page))
+			return NULL;
+
+		/*
 		 * When pinning a compound page of order > 1 (which is what
 		 * hpage_pincount_available() checks for), use an exact count to
 		 * track it, via hpage_pincount_add/_sub().
--
2.7.5

Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Christoph Hellwig]

On Tue, Mar 17, 2020 at 07:47:32PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
> going to be given to hardware and can't move. It would truncate CMA
> permanently and should be excluded.
> 
> In gup slow path, slow path, where
> __gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
> but in fast path, there lacks such a check, which means a possible leak of
> CMA page to longterm pinned.
> 
> Place a check in try_grab_compound_head() in the fast path to fix the leak,
> and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
> migrate the page.
> 
> 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)

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Jason Gunthorpe]

On Tue, Mar 17, 2020 at 07:47:32PM +0800, Pingfan Liu wrote:
> FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
> going to be given to hardware and can't move. It would truncate CMA
> permanently and should be excluded.
> 
> In gup slow path, slow path, where
> __gup_longterm_locked->check_and_migrate_cma_pages() handles FOLL_LONGTERM,
> but in fast path, there lacks such a check, which means a possible leak of
> CMA page to longterm pinned.
> 
> Place a check in try_grab_compound_head() in the fast path to fix the leak,
> and if FOLL_LONGTERM happens on CMA, it will fall back to slow path to
> migrate the page.
> 
> 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: Christoph Hellwig <hch@infradead.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Jason Gunthorpe <jgg@ziepe.ca>
> To: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> ---
> v6 -> v7: fix coding style issue
>  mm/gup.c | 9 +++++++++
>  1 file changed, 9 insertions(+)

Much clearer, thank you

Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>

Jason

Re: [PATCHv6 3/3] mm/gup_benchemark: add LONGTERM_BENCHMARK test in gup fast path

[Pingfan Liu]

On Fri, Mar 20, 2020 at 6:27 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 3/15/20 9:34 PM, Pingfan Liu wrote:
> > Introduce a PIN_FAST_LONGTERM_BENCHMARK ioctl to test longterm pin in gup fast
> > path.
>
> 1. The subject line still has "benchemark", which should be "benchmark".
>
> 2. What do you really want to test? More about the use case to be tested would help.
> Just another sentence. I wouldn't normally ask, but in this case the implementation
> is slightly scrambled, and I can't suggest how to fix it because there's no information
> in the commit log as to the use case, nor the motivation.
Oh, the history about [3/3] is to verify the implementation method of
[2/3]. Please refer to
https://lore.kernel.org/linux-mm/20190611122935.GA9919@dhcp-128-55.nay.redhat.com/
Cite "
> > I think the concern is: for the successful gup_fast case with no CMA

> > pages, this patch is adding another complete loop through all the
> > pages. In the fast case.
> >
> > If the check were instead done as part of the gup_pte_range(), then
> > it would be a little more efficient for that case.
> >
> > As for whether it's worth it, *probably* this is too small an effect to measure.
> > But in order to attempt a measurement: running fio (https://github.com/axboe/fio)
> > with O_DIRECT on an NVMe drive, might shed some light. Here's an fio.conf file
> > that Jan Kara and Tom Talpey helped me come up with, for related testing:
"
But I think now, there is no motivation for it, and can be dropped it now.

Thanks,
Pingfan

Re: [PATCHv7 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu]

On Fri, Mar 20, 2020 at 6:17 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 3/17/20 4:47 AM, Pingfan Liu wrote:
> > FOLL_LONGTERM is a special case of FOLL_PIN. It suggests a pin which is
> > going to be given to hardware and can't move. It would truncate CMA
> > permanently and should be excluded.
> >
> > In gup slow path, slow path, where
>
>
> s/slow path, slow path/slow path/
Yeah.
[...]
> >
> >               /*
> > +              * 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 subpage.
> > +              */
>
> Urggh, this comment is fine in the commit description, but at this location in the
> code it is completely incomprehensible! Instead of an extremely far-removed tidbit about
> interactions between CMA and huge pages, this comment should be explaining why we bail
> out early in the specific case of FOLL_PIN + FOLL_LONGTERM. And we don't bail out for
> FOLL_GET + FOLL_LONGTERM...
>
>
> I'm expect it is something like:
>
>                 /*
>                  * We can't do FOLL_LONGTERM + FOLL_PIN with CMA in the gup fast
>                  * path, so fail and let the caller fall back to the slow path.
>                  */
>
>
> ...approximately. Right?
Yes, right. And I think it is better to drop "We".

Thanks,
Pingfan