iov_iter: add helper to save iov_iter state

In an ideal world, when someone is passed an iov_iter and returns X bytes,
then X bytes would have been consumed/advanced from the iov_iter. But we
have use cases that always consume the entire iterator, a few examples
of that are iomap and bdev O_DIRECT. This means we cannot rely on the
state of the iov_iter once we've called ->read_iter() or ->write_iter().

This would be easier if we didn't always have to deal with truncate of
the iov_iter, as rewinding would be trivial without that. We recently
added a commit to track the truncate state, but that grew the iov_iter
by 8 bytes and wasn't the best solution.

Implement a helper to save enough of the iov_iter state to sanely restore
it after we've called the read/write iterator helpers. This currently
only works for IOVEC/BVEC/KVEC as that's all we need, support for other
iterator types are left as an exercise for the reader.

Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wiacKV4Gh-MYjteU0LwNBSGpWrK-Ov25HdqB1ewinrFPg@mail.gmail.com/
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 5265024..984c4ab 100644 (file)
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -27,6 +27,12 @@ enum iter_type {
        ITER_DISCARD,
 };
 
+struct iov_iter_state {
+       size_t iov_offset;
+       size_t count;
+       unsigned long nr_segs;
+};
+
 struct iov_iter {
        u8 iter_type;
        bool data_source;
@@ -55,6 +61,14 @@ static inline enum iter_type iov_iter_type(const struct iov_iter *i)
        return i->iter_type;
 }
 
+static inline void iov_iter_save_state(struct iov_iter *iter,
+                                      struct iov_iter_state *state)
+{
+       state->iov_offset = iter->iov_offset;
+       state->count = iter->count;
+       state->nr_segs = iter->nr_segs;
+}
+
 static inline bool iter_is_iovec(const struct iov_iter *i)
 {
        return iov_iter_type(i) == ITER_IOVEC;
@@ -233,6 +247,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i, struct page **pages,
 ssize_t iov_iter_get_pages_alloc(struct iov_iter *i, struct page ***pages,
                        size_t maxsize, size_t *start);
 int iov_iter_npages(const struct iov_iter *i, int maxpages);
+void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state);
 
 const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags);
 

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index f2d50d6..755c10c 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1972,3 +1972,39 @@ int import_single_range(int rw, void __user *buf, size_t len,
        return 0;
 }
 EXPORT_SYMBOL(import_single_range);
+
+/**
+ * iov_iter_restore() - Restore a &struct iov_iter to the same state as when
+ *     iov_iter_save_state() was called.
+ *
+ * @i: &struct iov_iter to restore
+ * @state: state to restore from
+ *
+ * Used after iov_iter_save_state() to bring restore @i, if operations may
+ * have advanced it.
+ *
+ * Note: only works on ITER_IOVEC, ITER_BVEC, and ITER_KVEC
+ */
+void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state)
+{
+       if (WARN_ON_ONCE(!iov_iter_is_bvec(i) && !iter_is_iovec(i)) &&
+                        !iov_iter_is_kvec(i))
+               return;
+       i->iov_offset = state->iov_offset;
+       i->count = state->count;
+       /*
+        * For the *vec iters, nr_segs + iov is constant - if we increment
+        * the vec, then we also decrement the nr_segs count. Hence we don't
+        * need to track both of these, just one is enough and we can deduct
+        * the other from that. ITER_KVEC and ITER_IOVEC are the same struct
+        * size, so we can just increment the iov pointer as they are unionzed.
+        * ITER_BVEC _may_ be the same size on some archs, but on others it is
+        * not. Be safe and handle it separately.
+        */
+       BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec));
+       if (iov_iter_is_bvec(i))
+               i->bvec -= state->nr_segs - i->nr_segs;
+       else
+               i->iov -= state->nr_segs - i->nr_segs;
+       i->nr_segs = state->nr_segs;
+}