[PATCH v8 05/17] iov_iter: Introduce fault_in_iov_iter_writeable

Andreas Gruenbacher
2021-10-19T13:42:53Z

Introduce a new fault_in_iov_iter_writeable helper for safely faulting
in an iterator for writing. Uses get_user_pages() to fault in the pages
without actually writing to them, which would be destructive.

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

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
include/linux/pagemap.h | 1 +
include/linux/uio.h | 1 +
lib/iov_iter.c | 39 +++++++++++++++++++++++++
mm/gup.c | 63 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 104 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 9fe94f7a4f7e..2f7dd14083d9 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -736,6 +736,7 @@ extern void add_page_wait_queue(struct page *page, wait_queue_entry_t *waiter);
* Fault in userspace address range.
*/
size_t fault_in_writeable(char __user *uaddr, size_t size);
+size_t fault_in_safe_writeable(const char __user *uaddr, size_t size);
size_t fault_in_readable(const char __user *uaddr, size_t size);

int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
diff --git a/include/linux/uio.h b/include/linux/uio.h
index d18458af6681..25d1c24fd829 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -134,6 +134,7 @@ 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);
size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t bytes);
+size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t bytes);
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 ce3d4f610626..ac9a87e727a3 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -467,6 +467,45 @@ size_t fault_in_iov_iter_readable(const struct iov_iter *i, size_t size)
}
EXPORT_SYMBOL(fault_in_iov_iter_readable);

+/*
+ * fault_in_iov_iter_writeable - fault in iov iterator for writing
+ * @i: iterator
+ * @size: maximum length
+ *
+ * Faults in the iterator using get_user_pages(), i.e., without triggering
+ * hardware page faults. This is primarily useful when we already know that
+ * some or all of the pages in @i aren't in memory.
+ *
+ * Returns the number of bytes not faulted in, like copy_to_user() and
+ * copy_from_user().
+ *
+ * Always returns 0 for non-user-space iterators.
+ */
+size_t fault_in_iov_iter_writeable(const struct iov_iter *i, size_t size)
+{
+ if (iter_is_iovec(i)) {
+ size_t count = min(size, iov_iter_count(i));
+ const struct iovec *p;
+ size_t skip;
+
+ size -= count;
+ for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
+ size_t len = min(count, p->iov_len - skip);
+ size_t ret;
+
+ if (unlikely(!len))
+ continue;
+ ret = fault_in_safe_writeable(p->iov_base + skip, len);
+ count -= len - ret;
+ if (ret)
+ break;
+ }
+ return count + size;
+ }
+ return 0;
+}
+EXPORT_SYMBOL(fault_in_iov_iter_writeable);
+
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 a7efb027d6cf..614b8536b3b6 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1691,6 +1691,69 @@ size_t fault_in_writeable(char __user *uaddr, size_t size)
}
EXPORT_SYMBOL(fault_in_writeable);

+/*
+ * fault_in_safe_writeable - fault in an address range for writing
+ * @uaddr: start of address range
+ * @size: length of address range
+ *
+ * Faults in an address range using get_user_pages, i.e., without triggering
+ * hardware page faults. This is primarily useful when we already know that
+ * some or all of the pages in the address range aren't in memory.
+ *
+ * Other than fault_in_writeable(), this function is non-destructive.
+ *
+ * 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 not faulted in, like copy_to_user() and
+ * copy_from_user().
+ */
+size_t fault_in_safe_writeable(const char __user *uaddr, size_t size)
+{
+ unsigned long start = (unsigned long)uaddr;
+ unsigned long end, nstart, nend;
+ struct mm_struct *mm = current->mm;
+ struct vm_area_struct *vma = NULL;
+ int locked = 0;
+
+ nstart = start & PAGE_MASK;
+ end = PAGE_ALIGN(start + size);
+ if (end < nstart)
+ end = 0;
+ for (; 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 = end ? min(end, vma->vm_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,
+ FOLL_TOUCH | FOLL_WRITE);
+ if (ret <= 0)
+ break;
+ nend = nstart + ret * PAGE_SIZE;
+ }
+ if (locked)
+ mmap_read_unlock(mm);
+ if (nstart == end)
+ return 0;
+ return size - min_t(size_t, nstart - start, size);
+}
+EXPORT_SYMBOL(fault_in_safe_writeable);
+
/**
* fault_in_readable - fault in userspace address range for reading
* @uaddr: start of user address range
--
2.26.3