[PATCH v3 1/7] iov_iter: Introduce fault_in_iov_iter helper

[Andreas Gruenbacher <agruenba@redhat.com>]

Introduce a new fault_in_iov_iter helper for manually faulting in an iterator.
Other than fault_in_pages_writeable(), this function is non-destructive.

We'll use fault_in_iov_iter in gfs2 once we've determined that the iterator
passed to .read_iter or .write_iter isn't in memory.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 include/linux/mm.h  |  3 ++
 include/linux/uio.h |  1 +
 lib/iov_iter.c      | 42 ++++++++++++++++++++++++++++
 mm/gup.c            | 68 +++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 114 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7ca22e6e694a..14b1353995e2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1840,6 +1840,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
 int pin_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages);
 
+unsigned long fault_in_user_pages(unsigned long start, unsigned long len,
+				  bool write);
+
 int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc);
 int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
 			struct task_struct *task, bool bypass_rlim);
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 82c3c3e819e0..152b3605e86c 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -120,6 +120,7 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
 void iov_iter_advance(struct iov_iter *i, size_t bytes);
 void iov_iter_revert(struct iov_iter *i, size_t bytes);
 int iov_iter_fault_in_readable(const struct iov_iter *i, size_t bytes);
+size_t fault_in_iov_iter(const struct iov_iter *i);
 size_t iov_iter_single_seg_count(const struct iov_iter *i);
 size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 20dc3d800573..7221665f7ac4 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -460,6 +460,48 @@ int iov_iter_fault_in_readable(const struct iov_iter *i, size_t bytes)
 }
 EXPORT_SYMBOL(iov_iter_fault_in_readable);
 
+/**
+ * fault_in_iov_iter - fault in iov iterator for reading / writing
+ * @i: iterator
+ *
+ * Faults in the iterator using get_user_pages, i.e., without triggering
+ * hardware page faults.
+ *
+ * This is primarily useful when we know that some or all of the pages in @i
+ * aren't in memory.  For iterators that are likely to be in memory,
+ * fault_in_pages_readable() may be more appropriate.
+ *
+ * Other than fault_in_pages_writeable(), this function is non-destructive even
+ * when faulting in pages for writing.
+ *
+ * Returns the number of bytes faulted in, or the size of @i if @i doesn't need
+ * faulting in.
+ */
+size_t fault_in_iov_iter(const struct iov_iter *i)
+{
+	size_t count = i->count;
+	const struct iovec *p;
+	size_t ret = 0, skip;
+
+	if (iter_is_iovec(i)) {
+		for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
+			unsigned long len = min(count, p->iov_len - skip);
+			unsigned long start, l;
+
+			if (unlikely(!len))
+				continue;
+			start = (unsigned long)p->iov_base + skip;
+			l = fault_in_user_pages(start, len, iov_iter_rw(i) != WRITE);
+			ret += l;
+			if (unlikely(l != len))
+				break;
+			count -= l;
+		}
+	}
+	return ret;
+}
+EXPORT_SYMBOL(fault_in_iov_iter);
+
 void iov_iter_init(struct iov_iter *i, unsigned int direction,
 			const struct iovec *iov, unsigned long nr_segs,
 			size_t count)
diff --git a/mm/gup.c b/mm/gup.c
index 42b8b1fa6521..033d66586c62 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1669,6 +1669,74 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
 }
 #endif /* !CONFIG_MMU */
 
+/**
+ * fault_in_user_pages - fault in an address range for reading / writing
+ * @start: start of address range
+ * @len: length of address range
+ * @write: fault in for writing
+ *
+ * Note that we don't pin or otherwise hold the pages referenced that we fault
+ * in.  There's no guarantee that they'll stay in memory for any duration of
+ * time.
+ *
+ * Returns the number of bytes faulted in from @start.
+ */
+unsigned long fault_in_user_pages(unsigned long start, unsigned long len,
+				  bool write)
+{
+	struct mm_struct *mm = current->mm;
+	struct vm_area_struct *vma = NULL;
+	unsigned long end, nstart, nend;
+	int locked = 0;
+	int gup_flags;
+
+	/*
+	 * FIXME: Make sure this function doesn't succeed for pages that cannot
+	 * be accessed; otherwise we could end up in a loop trying to fault in
+	 * and then access the pages.  (It's okay if a page gets evicted and we
+	 * need more than one retry.)
+	 */
+
+	/*
+	 * FIXME: Are these the right FOLL_* flags?
+	 */
+
+	gup_flags = FOLL_TOUCH | FOLL_POPULATE;
+	if (write)
+		gup_flags |= FOLL_WRITE;
+
+	end = PAGE_ALIGN(start + len);
+	for (nstart = start & PAGE_MASK; nstart < end; nstart = nend) {
+		unsigned long nr_pages;
+		long ret;
+
+		if (!locked) {
+			locked = 1;
+			mmap_read_lock(mm);
+			vma = find_vma(mm, nstart);
+		} else if (nstart >= vma->vm_end)
+			vma = vma->vm_next;
+		if (!vma || vma->vm_start >= end)
+			break;
+		nend = min(end, vma->vm_end);
+		if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+			continue;
+		if (nstart < vma->vm_start)
+			nstart = vma->vm_start;
+		nr_pages = (nend - nstart) / PAGE_SIZE;
+		ret = __get_user_pages_locked(mm, nstart, nr_pages,
+					      NULL, NULL, &locked, gup_flags);
+		if (ret <= 0)
+			break;
+		nend = nstart + ret * PAGE_SIZE;
+	}
+	if (locked)
+		mmap_read_unlock(mm);
+	if (nstart > start)
+		return min(nstart - start, len);
+	return 0;
+}
+
 /**
  * get_dump_page() - pin user page in memory while writing it to core dump
  * @addr: user address
-- 
2.26.3