[v6,05/17] mm/frame-vector: Use FOLL_LONGTERM

Commit Message

Daniel Vetter Nov. 19, 2020, 2:41 p.m. UTC

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

Note that pin_user_pages_fast is a safe replacement despite the
seeming lack of checking for vma->vm_flasg & (VM_IO | VM_PFNMAP). Such
ptes are marked with pte_mkspecial (which pup_fast rejects in the
fastpath), and only architectures supporting that support the
pin_user_pages_fast fastpath.

Signed-off-by: Daniel Vetter <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Pawel Osciak <[e-mail]>
Cc: Marek Szyprowski <[e-mail]>
Cc: Kyungmin Park <[e-mail]>
Cc: Tomasz Figa <[e-mail]>
Cc: Mauro Carvalho Chehab <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Jérôme Glisse <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Daniel Vetter <[e-mail]>
--
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

v6: Explain why pup_fast is safe, after discussions with John and
Christoph.
---
 .../media/common/videobuf2/videobuf2-memops.c |  3 +-
 include/linux/mm.h                            |  2 +-
 mm/frame_vector.c                             | 53 ++++++-------------
 3 files changed, 19 insertions(+), 39 deletions(-)

Patch

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 db6ae4d3fb4e..efb8c39bc933 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))) {
+	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)