Re: [git pull] iov_iter fixes

[Jens Axboe]

On 9/10/21 9:04 AM, Jens Axboe wrote:
> We could pretty this up and have the state part be explicit in iov_iter,
> and have the store/restore parts end up in uio.h. That'd tie them closer
> together, though I don't expect iov_iter changes to be an issue. It
> would make it more maintainable, though. I'll try and hack up this
> generic solution, see if that looks any better.

Looks something like this. Not super pretty in terms of needing a define
for this, and maybe I'm missing something, but ideally we'd want it as
an anonymous struct that's defined inside iov_iter. Anyway, gets the
point across. Alternatively, since we're down to just a few members now,
we just duplicate them in each struct...

Would be split into two patches, one for the iov_state addition and
the save/restore helpers, and then one switching io_uring to use them.
Figured we'd need some agreement on this first...


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 855ea544807f..539c94299d64 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2608,8 +2608,6 @@ static bool io_resubmit_prep(struct io_kiocb *req)
 
 	if (!rw)
 		return !io_req_prep_async(req);
-	/* may have left rw->iter inconsistent on -EIOCBQUEUED */
-	iov_iter_revert(&rw->iter, req->result - iov_iter_count(&rw->iter));
 	return true;
 }
 
@@ -3431,14 +3429,23 @@ static bool need_read_all(struct io_kiocb *req)
 		S_ISBLK(file_inode(req->file)->i_mode);
 }
 
+static void io_iter_restore(struct iov_iter *iter, struct iov_iter_state *state,
+			    ssize_t did_bytes)
+{
+	iov_iter_restore_state(iter, state);
+	if (did_bytes > 0)
+		iov_iter_advance(iter, did_bytes);
+}
+
 static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 {
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
 	struct kiocb *kiocb = &req->rw.kiocb;
 	struct iov_iter __iter, *iter = &__iter;
 	struct io_async_rw *rw = req->async_data;
-	ssize_t io_size, ret, ret2;
 	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
+	struct iov_iter_state state;
+	ssize_t ret, ret2;
 
 	if (rw) {
 		iter = &rw->iter;
@@ -3448,8 +3455,8 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 		if (ret < 0)
 			return ret;
 	}
-	io_size = iov_iter_count(iter);
-	req->result = io_size;
+	req->result = iov_iter_count(iter);
+	iov_iter_save_state(iter, &state);
 
 	/* Ensure we clear previously set non-block flag */
 	if (!force_nonblock)
@@ -3463,7 +3470,7 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 		return ret ?: -EAGAIN;
 	}
 
-	ret = rw_verify_area(READ, req->file, io_kiocb_ppos(kiocb), io_size);
+	ret = rw_verify_area(READ, req->file, io_kiocb_ppos(kiocb), state.count);
 	if (unlikely(ret)) {
 		kfree(iovec);
 		return ret;
@@ -3479,18 +3486,17 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 		/* no retry on NONBLOCK nor RWF_NOWAIT */
 		if (req->flags & REQ_F_NOWAIT)
 			goto done;
-		/* some cases will consume bytes even on error returns */
-		iov_iter_reexpand(iter, iter->count + iter->truncated);
-		iov_iter_revert(iter, io_size - iov_iter_count(iter));
 		ret = 0;
 	} else if (ret == -EIOCBQUEUED) {
 		goto out_free;
-	} else if (ret <= 0 || ret == io_size || !force_nonblock ||
+	} else if (ret <= 0 || ret == state.count || !force_nonblock ||
 		   (req->flags & REQ_F_NOWAIT) || !need_read_all(req)) {
 		/* read all, failed, already did sync or don't want to retry */
 		goto done;
 	}
 
+	io_iter_restore(iter, &state, ret);
+
 	ret2 = io_setup_async_rw(req, iovec, inline_vecs, iter, true);
 	if (ret2)
 		return ret2;
@@ -3501,7 +3507,7 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 	iter = &rw->iter;
 
 	do {
-		io_size -= ret;
+		state.count -= ret;
 		rw->bytes_done += ret;
 		/* if we can retry, do so with the callbacks armed */
 		if (!io_rw_should_retry(req)) {
@@ -3520,7 +3526,7 @@ static int io_read(struct io_kiocb *req, unsigned int issue_flags)
 			return 0;
 		/* we got some bytes, but not all. retry. */
 		kiocb->ki_flags &= ~IOCB_WAITQ;
-	} while (ret > 0 && ret < io_size);
+	} while (ret > 0 && ret < state.count);
 done:
 	kiocb_done(kiocb, ret, issue_flags);
 out_free:
@@ -3543,8 +3549,9 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
 	struct kiocb *kiocb = &req->rw.kiocb;
 	struct iov_iter __iter, *iter = &__iter;
 	struct io_async_rw *rw = req->async_data;
-	ssize_t ret, ret2, io_size;
 	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
+	struct iov_iter_state state;
+	ssize_t ret, ret2;
 
 	if (rw) {
 		iter = &rw->iter;
@@ -3554,8 +3561,9 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
 		if (ret < 0)
 			return ret;
 	}
-	io_size = iov_iter_count(iter);
-	req->result = io_size;
+	req->result = iov_iter_count(iter);
+	iov_iter_save_state(iter, &state);
+	ret2 = 0;
 
 	/* Ensure we clear previously set non-block flag */
 	if (!force_nonblock)
@@ -3572,7 +3580,7 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
 	    (req->flags & REQ_F_ISREG))
 		goto copy_iov;
 
-	ret = rw_verify_area(WRITE, req->file, io_kiocb_ppos(kiocb), io_size);
+	ret = rw_verify_area(WRITE, req->file, io_kiocb_ppos(kiocb), state.count);
 	if (unlikely(ret))
 		goto out_free;
 
@@ -3619,9 +3627,7 @@ static int io_write(struct io_kiocb *req, unsigned int issue_flags)
 		kiocb_done(kiocb, ret2, issue_flags);
 	} else {
 copy_iov:
-		/* some cases will consume bytes even on error returns */
-		iov_iter_reexpand(iter, iter->count + iter->truncated);
-		iov_iter_revert(iter, io_size - iov_iter_count(iter));
+		io_iter_restore(iter, &state, ret2);
 		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
 		return ret ?: -EAGAIN;
 	}
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 5265024e8b90..4f9d483096cd 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -27,11 +27,25 @@ enum iter_type {
 	ITER_DISCARD,
 };
 
+#define IOV_ITER_STATE					\
+	size_t iov_offset;				\
+	size_t count;					\
+	union {						\
+		unsigned long nr_segs;			\
+		struct {				\
+			unsigned int head;		\
+			unsigned int start_head;	\
+		};					\
+		loff_t xarray_start;			\
+	};						\
+
+struct iov_iter_state {
+	IOV_ITER_STATE;
+};
+
 struct iov_iter {
 	u8 iter_type;
 	bool data_source;
-	size_t iov_offset;
-	size_t count;
 	union {
 		const struct iovec *iov;
 		const struct kvec *kvec;
@@ -40,12 +54,10 @@ struct iov_iter {
 		struct pipe_inode_info *pipe;
 	};
 	union {
-		unsigned long nr_segs;
+		struct iov_iter_state state;
 		struct {
-			unsigned int head;
-			unsigned int start_head;
+			IOV_ITER_STATE;
 		};
-		loff_t xarray_start;
 	};
 	size_t truncated;
 };
@@ -55,6 +67,33 @@ 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 = iter->state;
+}
+
+static inline void iov_iter_restore_state(struct iov_iter *iter,
+					  struct iov_iter_state *state)
+{
+	iter->iov_offset = state->iov_offset;
+	iter->count = state->count;
+
+	switch (iov_iter_type(iter)) {
+	case ITER_IOVEC:
+	case ITER_KVEC:
+	case ITER_BVEC:
+		iter->iov -= state->nr_segs - iter->nr_segs;
+		fallthrough;
+	case ITER_PIPE:
+	case ITER_XARRAY:
+		iter->nr_segs = state->nr_segs;
+		break;
+	case ITER_DISCARD:
+		break;
+	}
+}
+
 static inline bool iter_is_iovec(const struct iov_iter *i)
 {
 	return iov_iter_type(i) == ITER_IOVEC;

-- 
Jens Axboe