Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

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On 10/30/20 3:08 AM, Daniel Vetter wrote:
This is used by media/videbuf2 for persistent dma mappings, not just
for a single dma operation and then freed again, so needs
FOLL_LONGTERM.

Unfortunately current pup_locked doesn't support FOLL_LONGTERM due to
locking issues. Rework the code to pull the pup path out from the
mmap_sem critical section as suggested by Jason.

By relying entirely on the vma checks in pin_user_pages and follow_pfn

There are vma checks in pin_user_pages(), but this patch changes things
to call pin_user_pages_fast(). And that does not have the vma checks.
More below about this:

(for vm_flags and vma_is_fsdax) we can also streamline the code a lot.

Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Pawel Osciak <pawel@xxxxxxxxxx>
Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Cc: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
Cc: Tomasz Figa <tfiga@xxxxxxxxxxxx>
Cc: Mauro Carvalho Chehab <mchehab@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Jérôme Glisse <jglisse@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: linux-samsung-soc@xxxxxxxxxxxxxxx
Cc: linux-media@xxxxxxxxxxxxxxx
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
--
v2: Streamline the code and further simplify the loop checks (Jason)

v5: Review from Tomasz:
- fix page counting for the follow_pfn case by resetting ret
- drop gup_flags paramater, now unused
---
  .../media/common/videobuf2/videobuf2-memops.c |  3 +-
  include/linux/mm.h                            |  2 +-
  mm/frame_vector.c                             | 53 ++++++-------------
  3 files changed, 19 insertions(+), 39 deletions(-)

diff --git a/drivers/media/common/videobuf2/videobuf2-memops.c b/drivers/media/common/videobuf2/videobuf2-memops.c
index 6e9e05153f4e..9dd6c27162f4 100644
--- a/drivers/media/common/videobuf2/videobuf2-memops.c
+++ b/drivers/media/common/videobuf2/videobuf2-memops.c
@@ -40,7 +40,6 @@ struct frame_vector *vb2_create_framevec(unsigned long start,
  	unsigned long first, last;
  	unsigned long nr;
  	struct frame_vector *vec;
-	unsigned int flags = FOLL_FORCE | FOLL_WRITE;
first = start >> PAGE_SHIFT;
  	last = (start + length - 1) >> PAGE_SHIFT;
@@ -48,7 +47,7 @@ struct frame_vector *vb2_create_framevec(unsigned long start,
  	vec = frame_vector_create(nr);
  	if (!vec)
  		return ERR_PTR(-ENOMEM);
-	ret = get_vaddr_frames(start & PAGE_MASK, nr, flags, vec);
+	ret = get_vaddr_frames(start & PAGE_MASK, nr, vec);
  	if (ret < 0)
  		goto out_destroy;
  	/* We accept only complete set of PFNs */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ef360fe70aaf..d6b8e30dce2e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1765,7 +1765,7 @@ struct frame_vector {
  struct frame_vector *frame_vector_create(unsigned int nr_frames);
  void frame_vector_destroy(struct frame_vector *vec);
  int get_vaddr_frames(unsigned long start, unsigned int nr_pfns,
-		     unsigned int gup_flags, struct frame_vector *vec);
+		     struct frame_vector *vec);
  void put_vaddr_frames(struct frame_vector *vec);
  int frame_vector_to_pages(struct frame_vector *vec);
  void frame_vector_to_pfns(struct frame_vector *vec);
diff --git a/mm/frame_vector.c b/mm/frame_vector.c
index 10f82d5643b6..f8c34b895c76 100644
--- a/mm/frame_vector.c
+++ b/mm/frame_vector.c
@@ -32,13 +32,12 @@
   * This function takes care of grabbing mmap_lock as necessary.
   */
  int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
-		     unsigned int gup_flags, struct frame_vector *vec)
+		     struct frame_vector *vec)
  {
  	struct mm_struct *mm = current->mm;
  	struct vm_area_struct *vma;
  	int ret = 0;
  	int err;
-	int locked;
if (nr_frames == 0)
  		return 0;
@@ -48,40 +47,26 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
start = untagged_addr(start); - mmap_read_lock(mm);
-	locked = 1;
-	vma = find_vma_intersection(mm, start, start + 1);
-	if (!vma) {
-		ret = -EFAULT;
-		goto out;
-	}
-
-	/*
-	 * While get_vaddr_frames() could be used for transient (kernel
-	 * controlled lifetime) pinning of memory pages all current
-	 * users establish long term (userspace controlled lifetime)
-	 * page pinning. Treat get_vaddr_frames() like
-	 * get_user_pages_longterm() and disallow it for filesystem-dax
-	 * mappings.
-	 */
-	if (vma_is_fsdax(vma)) {
-		ret = -EOPNOTSUPP;
-		goto out;
-	}
-
-	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {

By removing this check from this location, and changing from
pin_user_pages_locked() to pin_user_pages_fast(), I *think* we end up
losing the check entirely. Is that intended? If so it could use a comment
somewhere to explain why.

thanks,
--
John Hubbard
NVIDIA

+	ret = pin_user_pages_fast(start, nr_frames,
+				  FOLL_FORCE | FOLL_WRITE | FOLL_LONGTERM,
+				  (struct page **)(vec->ptrs));
+	if (ret > 0) {
  		vec->got_ref = true;
  		vec->is_pfns = false;
-		ret = pin_user_pages_locked(start, nr_frames,
-			gup_flags, (struct page **)(vec->ptrs), &locked);
-		goto out;
+		goto out_unlocked;
  	}
+ mmap_read_lock(mm);
  	vec->got_ref = false;
  	vec->is_pfns = true;
+	ret = 0;
  	do {
  		unsigned long *nums = frame_vector_pfns(vec);
+ vma = find_vma_intersection(mm, start, start + 1);
+		if (!vma)
+			break;
+
  		while (ret < nr_frames && start + PAGE_SIZE <= vma->vm_end) {
  			err = follow_pfn(vma, start, &nums[ret]);
  			if (err) {
@@ -92,17 +77,13 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
  			start += PAGE_SIZE;
  			ret++;
  		}
-		/*
-		 * We stop if we have enough pages or if VMA doesn't completely
-		 * cover the tail page.
-		 */
-		if (ret >= nr_frames || start < vma->vm_end)
+		/* Bail out if VMA doesn't completely cover the tail page. */
+		if (start < vma->vm_end)
  			break;
-		vma = find_vma_intersection(mm, start, start + 1);
-	} while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
+	} while (ret < nr_frames);
  out:
-	if (locked)
-		mmap_read_unlock(mm);
+	mmap_read_unlock(mm);
+out_unlocked:
  	if (!ret)
  		ret = -EFAULT;
  	if (ret > 0)







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