iov_iter: add ITER_BVEC_FLAG_NO_REF flag

For ITER_BVEC, if we're holding on to kernel pages, the caller
doesn't need to grab a reference to the bvec pages, and drop that
same reference on IO completion. This is essentially safe for any
ITER_BVEC, but some use cases end up reusing pages and uncondtionally
dropping a page reference on completion. And example of that is
sendfile(2), that ends up being a splice_in + splice_out on the
pipe pages.

Add a flag that tells us it's fine to not grab a page reference
to the bvec pages, since that caller knows not to drop a reference
when it's done with the pages.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -855,6 +855,9 @@ static int io_import_fixed(struct io_ring_ctx *ctx, int rw,
 	iov_iter_bvec(iter, rw, imu->bvec, imu->nr_bvecs, offset + len);
 	if (offset)
 		iov_iter_advance(iter, offset);
+
+	/* don't drop a reference to these pages */
+	iter->type |= ITER_BVEC_FLAG_NO_REF;
 	return 0;
 }
 

include/linux/uio.h

@@ -23,14 +23,23 @@ struct kvec {
 };
 
 enum iter_type {
-	ITER_IOVEC = 0,
-	ITER_KVEC = 2,
-	ITER_BVEC = 4,
-	ITER_PIPE = 8,
-	ITER_DISCARD = 16,
+	/* set if ITER_BVEC doesn't hold a bv_page ref */
+	ITER_BVEC_FLAG_NO_REF = 2,
+
+	/* iter types */
+	ITER_IOVEC = 4,
+	ITER_KVEC = 8,
+	ITER_BVEC = 16,
+	ITER_PIPE = 32,
+	ITER_DISCARD = 64,
 };
 
 struct iov_iter {
+	/*
+	 * Bit 0 is the read/write bit, set if we're writing.
+	 * Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and
+	 * the caller isn't expecting to drop a page reference when done.
+	 */
 	unsigned int type;
 	size_t iov_offset;
 	size_t count;
@@ -84,6 +93,11 @@ static inline unsigned char iov_iter_rw(const struct iov_iter *i)
 	return i->type & (READ | WRITE);
 }
 
+static inline bool iov_iter_bvec_no_ref(const struct iov_iter *i)
+{
+	return (i->type & ITER_BVEC_FLAG_NO_REF) != 0;
+}
+
 /*
  * Total number of bytes covered by an iovec.
  *