[PATCH 8/8] aio: support for IO polling

[Jens Axboe <axboe@kernel.dk>]

Add polled variants of PREAD/PREADV and PWRITE/PWRITEV. These act
like their non-polled counterparts, except we expect to poll for
completion of them. The polling happens at io_getevent() time, and
works just like non-polled IO.

To setup an io_context for polled IO, the application must call
io_setup2() with IOCTX_FLAG_IOPOLL as one of the flags. It is illegal
to mix and match polled and non-polled IO on an io_context.

Polled IO doesn't support the user mapped completion ring. Events
must be reaped through the io_getevents() system call. For non-irq
driven poll devices, there's no way to support completion reaping
from userspace by just looking at the ring. The application itself
is the one that pulls completion entries.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/aio.c                     | 377 ++++++++++++++++++++++++++++++-----
 include/uapi/linux/aio_abi.h |   4 +
 2 files changed, 336 insertions(+), 45 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 8bbb0b77d9c4..ea93847d25d1 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -94,6 +94,8 @@ struct kioctx {
 
 	unsigned long		user_id;
 
+	unsigned int		flags;
+
 	struct __percpu kioctx_cpu *cpu;
 
 	/*
@@ -138,6 +140,19 @@ struct kioctx {
 		atomic_t	reqs_available;
 	} ____cacheline_aligned_in_smp;
 
+	/* iopoll submission state */
+	struct {
+		spinlock_t poll_lock;
+		struct list_head poll_submitted;
+	} ____cacheline_aligned_in_smp;
+
+	/* iopoll completion state */
+	struct {
+		struct list_head poll_completing;
+		unsigned long getevents_busy;
+		atomic_t poll_completed;
+	} ____cacheline_aligned_in_smp;
+
 	struct {
 		spinlock_t	ctx_lock;
 		struct list_head active_reqs;	/* used for cancellation */
@@ -191,13 +206,24 @@ struct aio_kiocb {
 
 	struct list_head	ki_list;	/* the aio core uses this
 						 * for cancellation */
+
+	unsigned long		ki_flags;
+#define IOCB_POLL_COMPLETED	0
+
 	refcount_t		ki_refcnt;
 
-	/*
-	 * If the aio_resfd field of the userspace iocb is not zero,
-	 * this is the underlying eventfd context to deliver events to.
-	 */
-	struct eventfd_ctx	*ki_eventfd;
+	union {
+		/*
+		 * If the aio_resfd field of the userspace iocb is not zero,
+		 * this is the underlying eventfd context to deliver events to.
+		 */
+		struct eventfd_ctx	*ki_eventfd;
+
+		/*
+		 * For polled IO, stash completion info here
+		 */
+		struct io_event		ki_ev;
+	};
 };
 
 /*------ sysctl variables----*/
@@ -214,6 +240,8 @@ static struct vfsmount *aio_mnt;
 static const struct file_operations aio_ring_fops;
 static const struct address_space_operations aio_ctx_aops;
 
+static void aio_iopoll_reap_events(struct kioctx *);
+
 static struct file *aio_private_file(struct kioctx *ctx, loff_t nr_pages)
 {
 	struct file *file;
@@ -451,11 +479,15 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
 	int i;
 	struct file *file;
 
-	/* Compensate for the ring buffer's head/tail overlap entry */
-	nr_events += 2;	/* 1 is required, 2 for good luck */
-
+	/*
+	 * Compensate for the ring buffer's head/tail overlap entry.
+	 * IO polling doesn't require any io event entries
+	 */
 	size = sizeof(struct aio_ring);
-	size += sizeof(struct io_event) * nr_events;
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL)) {
+		nr_events += 2;	/* 1 is required, 2 for good luck */
+		size += sizeof(struct io_event) * nr_events;
+	}
 
 	nr_pages = PFN_UP(size);
 	if (nr_pages < 0)
@@ -720,6 +752,7 @@ static struct kioctx *ioctx_alloc(unsigned nr_events, unsigned int flags)
 	if (!ctx)
 		return ERR_PTR(-ENOMEM);
 
+	ctx->flags = flags;
 	ctx->max_reqs = max_reqs;
 
 	spin_lock_init(&ctx->ctx_lock);
@@ -732,6 +765,11 @@ static struct kioctx *ioctx_alloc(unsigned nr_events, unsigned int flags)
 
 	INIT_LIST_HEAD(&ctx->active_reqs);
 
+	spin_lock_init(&ctx->poll_lock);
+	INIT_LIST_HEAD(&ctx->poll_submitted);
+	INIT_LIST_HEAD(&ctx->poll_completing);
+	atomic_set(&ctx->poll_completed, 0);
+
 	if (percpu_ref_init(&ctx->users, free_ioctx_users, 0, GFP_KERNEL))
 		goto err;
 
@@ -814,6 +852,8 @@ static int kill_ioctx(struct mm_struct *mm, struct kioctx *ctx,
 	RCU_INIT_POINTER(table->table[ctx->id], NULL);
 	spin_unlock(&mm->ioctx_lock);
 
+	aio_iopoll_reap_events(ctx);
+
 	/* free_ioctx_reqs() will do the necessary RCU synchronization */
 	wake_up_all(&ctx->wait);
 
@@ -1056,6 +1096,24 @@ static inline void iocb_put(struct aio_kiocb *iocb)
 	}
 }
 
+static void iocb_put_many(struct kioctx *ctx, void **iocbs, int *nr)
+{
+	if (nr) {
+		kmem_cache_free_bulk(kiocb_cachep, *nr, iocbs);
+		percpu_ref_put_many(&ctx->reqs, *nr);
+		*nr = 0;
+	}
+}
+
+static void aio_fill_event(struct io_event *ev, struct aio_kiocb *iocb,
+			   long res, long res2)
+{
+	ev->obj = (u64)(unsigned long)iocb->ki_user_iocb;
+	ev->data = iocb->ki_user_data;
+	ev->res = res;
+	ev->res2 = res2;
+}
+
 /* aio_complete
  *	Called when the io request on the given iocb is complete.
  */
@@ -1083,10 +1141,7 @@ static void aio_complete(struct aio_kiocb *iocb, long res, long res2)
 	ev_page = kmap_atomic(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
 	event = ev_page + pos % AIO_EVENTS_PER_PAGE;
 
-	event->obj = (u64)(unsigned long)iocb->ki_user_iocb;
-	event->data = iocb->ki_user_data;
-	event->res = res;
-	event->res2 = res2;
+	aio_fill_event(event, iocb, res, res2);
 
 	kunmap_atomic(ev_page);
 	flush_dcache_page(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
@@ -1239,6 +1294,165 @@ static bool aio_read_events(struct kioctx *ctx, long min_nr, long nr,
 	return ret < 0 || *i >= min_nr;
 }
 
+#define AIO_POLL_STACK	8
+
+/*
+ * Process completed iocb iopoll entries, copying the result to userspace.
+ */
+static long aio_iopoll_reap(struct kioctx *ctx, struct io_event __user *evs,
+			    unsigned int *nr_events, long max)
+{
+	void *iocbs[AIO_POLL_STACK];
+	struct aio_kiocb *iocb, *n;
+	int to_free = 0, ret = 0;
+
+	list_for_each_entry_safe(iocb, n, &ctx->poll_completing, ki_list) {
+		if (!test_bit(IOCB_POLL_COMPLETED, &iocb->ki_flags))
+			continue;
+		if (to_free == AIO_POLL_STACK)
+			iocb_put_many(ctx, iocbs, &to_free);
+
+		list_del(&iocb->ki_list);
+		iocbs[to_free++] = iocb;
+
+		fput(iocb->rw.ki_filp);
+
+		if (!evs) {
+			(*nr_events)++;
+			continue;
+		}
+
+		if (copy_to_user(evs + *nr_events, &iocb->ki_ev,
+		    sizeof(iocb->ki_ev))) {
+			ret = -EFAULT;
+			break;
+		}
+		if (++(*nr_events) == max)
+			break;
+	}
+
+	if (to_free)
+		iocb_put_many(ctx, iocbs, &to_free);
+
+	return ret;
+}
+
+static int __aio_iopoll_check(struct kioctx *ctx, struct io_event __user *event,
+			      unsigned int *nr_events, long min, long max)
+{
+	struct aio_kiocb *iocb;
+	unsigned int poll_completed;
+	int to_poll, polled, ret;
+
+	/*
+	 * Check if we already have done events that satisfy what we need
+	 */
+	if (!list_empty(&ctx->poll_completing)) {
+		ret = aio_iopoll_reap(ctx, event, nr_events, max);
+		if (ret < 0)
+			return ret;
+		if (*nr_events >= min)
+			return 0;
+	}
+
+	/*
+	 * Take in a new working set from the submitted list if possible.
+	 */
+	if (!list_empty_careful(&ctx->poll_submitted)) {
+		spin_lock(&ctx->poll_lock);
+		list_splice_init(&ctx->poll_submitted, &ctx->poll_completing);
+		spin_unlock(&ctx->poll_lock);
+	}
+
+	if (list_empty(&ctx->poll_completing))
+		return 0;
+
+	/*
+	 * Check again now that we have a new batch.
+	 */
+	ret = aio_iopoll_reap(ctx, event, nr_events, max);
+	if (ret < 0)
+		return ret;
+	if (*nr_events >= min)
+		return 0;
+
+	/*
+	 * Find up to 'max_nr' worth of events to poll for, including the
+	 * events we already successfully polled
+	 */
+	polled = to_poll = 0;
+	poll_completed = atomic_read(&ctx->poll_completed);
+	list_for_each_entry(iocb, &ctx->poll_completing, ki_list) {
+		/*
+		 * Poll for needed events with wait == true, anything after
+		 * that we just check if we have more, up to max.
+		 */
+		bool wait = polled + *nr_events >= min;
+		struct kiocb *kiocb = &iocb->rw;
+
+		if (test_bit(IOCB_POLL_COMPLETED, &iocb->ki_flags))
+			break;
+		if (++to_poll + *nr_events >= max)
+			break;
+
+		polled += kiocb->ki_filp->f_op->iopoll(kiocb, wait);
+		if (polled + *nr_events >= max)
+			break;
+		if (poll_completed != atomic_read(&ctx->poll_completed))
+			break;
+	}
+
+	ret = aio_iopoll_reap(ctx, event, nr_events, max);
+	if (ret < 0)
+		return ret;
+	if (*nr_events >= min)
+		return 0;
+	return to_poll;
+}
+
+/*
+ * We can't just wait for polled events to come to us, we have to actively
+ * find and complete them.
+ */
+static void aio_iopoll_reap_events(struct kioctx *ctx)
+{
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL))
+		return;
+
+	while (!list_empty_careful(&ctx->poll_submitted) ||
+	       !list_empty(&ctx->poll_completing)) {
+		unsigned int nr_events = 0;
+
+		__aio_iopoll_check(ctx, NULL, &nr_events, 1, UINT_MAX);
+	}
+}
+
+static int aio_iopoll_check(struct kioctx *ctx, long min_nr, long nr,
+			    struct io_event __user *event)
+{
+	unsigned int nr_events = 0;
+	int ret = 0;
+
+	/* * Only allow one thread polling at a time */
+	if (test_and_set_bit(0, &ctx->getevents_busy))
+		return -EBUSY;
+
+	while (!nr_events || !need_resched()) {
+		int tmin = 0;
+
+		if (nr_events < min_nr)
+			tmin = min_nr - nr_events;
+
+		ret = __aio_iopoll_check(ctx, event, &nr_events, tmin, nr);
+		if (ret <= 0)
+			break;
+		ret = 0;
+	}
+
+	clear_bit(0, &ctx->getevents_busy);
+	return nr_events ? nr_events : ret;
+}
+
 static long read_events(struct kioctx *ctx, long min_nr, long nr,
 			struct io_event __user *event,
 			ktime_t until)
@@ -1287,7 +1501,7 @@ SYSCALL_DEFINE3(io_setup2, u32, nr_events, u32, flags,
 	unsigned long ctx;
 	long ret;
 
-	if (flags)
+	if (flags & ~IOCTX_FLAG_IOPOLL)
 		return -EINVAL;
 
 	ret = get_user(ctx, ctxp);
@@ -1411,13 +1625,8 @@ static void aio_remove_iocb(struct aio_kiocb *iocb)
 	spin_unlock_irqrestore(&ctx->ctx_lock, flags);
 }
 
-static void aio_complete_rw(struct kiocb *kiocb, long res, long res2)
+static void kiocb_end_write(struct kiocb *kiocb)
 {
-	struct aio_kiocb *iocb = container_of(kiocb, struct aio_kiocb, rw);
-
-	if (!list_empty_careful(&iocb->ki_list))
-		aio_remove_iocb(iocb);
-
 	if (kiocb->ki_flags & IOCB_WRITE) {
 		struct inode *inode = file_inode(kiocb->ki_filp);
 
@@ -1429,19 +1638,42 @@ static void aio_complete_rw(struct kiocb *kiocb, long res, long res2)
 			__sb_writers_acquired(inode->i_sb, SB_FREEZE_WRITE);
 		file_end_write(kiocb->ki_filp);
 	}
+}
+
+static void aio_complete_rw(struct kiocb *kiocb, long res, long res2)
+{
+	struct aio_kiocb *iocb = container_of(kiocb, struct aio_kiocb, rw);
+
+	if (!list_empty_careful(&iocb->ki_list))
+		aio_remove_iocb(iocb);
+
+	kiocb_end_write(kiocb);
 
 	fput(kiocb->ki_filp);
 	aio_complete(iocb, res, res2);
 }
 
-static int aio_prep_rw(struct kiocb *req, struct iocb *iocb)
+static void aio_complete_rw_poll(struct kiocb *kiocb, long res, long res2)
 {
+	struct aio_kiocb *iocb = container_of(kiocb, struct aio_kiocb, rw);
+	struct kioctx *ctx = iocb->ki_ctx;
+
+	kiocb_end_write(kiocb);
+
+	aio_fill_event(&iocb->ki_ev, iocb, res, res2);
+	set_bit(IOCB_POLL_COMPLETED, &iocb->ki_flags);
+	atomic_inc(&ctx->poll_completed);
+}
+
+static int aio_prep_rw(struct aio_kiocb *kiocb, struct iocb *iocb)
+{
+	struct kioctx *ctx = kiocb->ki_ctx;
+	struct kiocb *req = &kiocb->rw;
 	int ret;
 
 	req->ki_filp = fget(iocb->aio_fildes);
 	if (unlikely(!req->ki_filp))
 		return -EBADF;
-	req->ki_complete = aio_complete_rw;
 	req->ki_pos = iocb->aio_offset;
 	req->ki_flags = iocb_flags(req->ki_filp);
 	if (iocb->aio_flags & IOCB_FLAG_RESFD)
@@ -1456,8 +1688,7 @@ static int aio_prep_rw(struct kiocb *req, struct iocb *iocb)
 		ret = ioprio_check_cap(iocb->aio_reqprio);
 		if (ret) {
 			pr_debug("aio ioprio check cap error: %d\n", ret);
-			fput(req->ki_filp);
-			return ret;
+			goto out_fput;
 		}
 
 		req->ki_ioprio = iocb->aio_reqprio;
@@ -1466,7 +1697,41 @@ static int aio_prep_rw(struct kiocb *req, struct iocb *iocb)
 
 	ret = kiocb_set_rw_flags(req, iocb->aio_rw_flags);
 	if (unlikely(ret))
-		fput(req->ki_filp);
+		goto out_fput;
+
+	if (iocb->aio_flags & IOCB_FLAG_HIPRI) {
+		/* shares space in the union, and is rather pointless.. */
+		ret = -EINVAL;
+		if (iocb->aio_flags & IOCB_FLAG_RESFD)
+			goto out_fput;
+
+		/* can't submit polled IO to a non-polled ctx */
+		if (!(ctx->flags & IOCTX_FLAG_IOPOLL))
+			goto out_fput;
+
+		ret = -EOPNOTSUPP;
+		if (!(req->ki_flags & IOCB_DIRECT) ||
+		    !req->ki_filp->f_op->iopoll)
+			goto out_fput;
+
+		req->ki_flags |= IOCB_HIPRI;
+		req->ki_complete = aio_complete_rw_poll;
+
+		spin_lock(&ctx->poll_lock);
+		list_add_tail(&kiocb->ki_list, &ctx->poll_submitted);
+		spin_unlock(&ctx->poll_lock);
+	} else {
+		/* can't submit non-polled IO to a polled ctx */
+		ret = -EINVAL;
+		if (ctx->flags & IOCTX_FLAG_IOPOLL)
+			goto out_fput;
+
+		req->ki_complete = aio_complete_rw;
+	}
+
+	return 0;
+out_fput:
+	fput(req->ki_filp);
 	return ret;
 }
 
@@ -1509,15 +1774,16 @@ static inline void aio_rw_done(struct kiocb *req, ssize_t ret)
 	}
 }
 
-static ssize_t aio_read(struct kiocb *req, struct iocb *iocb, bool vectored,
-		bool compat)
+static ssize_t aio_read(struct aio_kiocb *kiocb, struct iocb *iocb,
+			bool vectored, bool compat)
 {
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
+	struct kiocb *req = &kiocb->rw;
 	struct iov_iter iter;
 	struct file *file;
 	ssize_t ret;
 
-	ret = aio_prep_rw(req, iocb);
+	ret = aio_prep_rw(kiocb, iocb);
 	if (ret)
 		return ret;
 	file = req->ki_filp;
@@ -1542,15 +1808,16 @@ static ssize_t aio_read(struct kiocb *req, struct iocb *iocb, bool vectored,
 	return ret;
 }
 
-static ssize_t aio_write(struct kiocb *req, struct iocb *iocb, bool vectored,
-		bool compat)
+static ssize_t aio_write(struct aio_kiocb *kiocb, struct iocb *iocb,
+			 bool vectored, bool compat)
 {
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
+	struct kiocb *req = &kiocb->rw;
 	struct iov_iter iter;
 	struct file *file;
 	ssize_t ret;
 
-	ret = aio_prep_rw(req, iocb);
+	ret = aio_prep_rw(kiocb, iocb);
 	if (ret)
 		return ret;
 	file = req->ki_filp;
@@ -1820,7 +2087,8 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 		return -EINVAL;
 	}
 
-	if (!get_reqs_available(ctx))
+	/* Poll IO doesn't need ring reservations */
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL) && !get_reqs_available(ctx))
 		return -EAGAIN;
 
 	ret = -EAGAIN;
@@ -1843,35 +2111,45 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 		}
 	}
 
-	ret = put_user(KIOCB_KEY, &user_iocb->aio_key);
-	if (unlikely(ret)) {
-		pr_debug("EFAULT: aio_key\n");
-		goto out_put_req;
+	/* polled IO isn't cancelable, don't bother copying the key */
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL)) {
+		ret = put_user(KIOCB_KEY, &user_iocb->aio_key);
+		if (unlikely(ret)) {
+			pr_debug("EFAULT: aio_key\n");
+			goto out_put_req;
+		}
 	}
 
 	req->ki_user_iocb = user_iocb;
 	req->ki_user_data = iocb.aio_data;
 
+	ret = -EINVAL;
 	switch (iocb.aio_lio_opcode) {
 	case IOCB_CMD_PREAD:
-		ret = aio_read(&req->rw, &iocb, false, compat);
+		ret = aio_read(req, &iocb, false, compat);
 		break;
 	case IOCB_CMD_PWRITE:
-		ret = aio_write(&req->rw, &iocb, false, compat);
+		ret = aio_write(req, &iocb, false, compat);
 		break;
 	case IOCB_CMD_PREADV:
-		ret = aio_read(&req->rw, &iocb, true, compat);
+		ret = aio_read(req, &iocb, true, compat);
 		break;
 	case IOCB_CMD_PWRITEV:
-		ret = aio_write(&req->rw, &iocb, true, compat);
+		ret = aio_write(req, &iocb, true, compat);
 		break;
 	case IOCB_CMD_FSYNC:
+		if (ctx->flags & IOCTX_FLAG_IOPOLL)
+			break;
 		ret = aio_fsync(&req->fsync, &iocb, false);
 		break;
 	case IOCB_CMD_FDSYNC:
+		if (ctx->flags & IOCTX_FLAG_IOPOLL)
+			break;
 		ret = aio_fsync(&req->fsync, &iocb, true);
 		break;
 	case IOCB_CMD_POLL:
+		if (ctx->flags & IOCTX_FLAG_IOPOLL)
+			break;
 		ret = aio_poll(req, &iocb);
 		break;
 	default:
@@ -1894,7 +2172,8 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 		eventfd_ctx_put(req->ki_eventfd);
 	kmem_cache_free(kiocb_cachep, req);
 out_put_reqs_available:
-	put_reqs_available(ctx, 1);
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL))
+		put_reqs_available(ctx, 1);
 	return ret;
 }
 
@@ -1930,7 +2209,9 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
 	if (nr > ctx->nr_events)
 		nr = ctx->nr_events;
 
-	blk_start_plug(&plug);
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL))
+		blk_start_plug(&plug);
+
 	for (i = 0; i < nr; i++) {
 		struct iocb __user *user_iocb;
 
@@ -1943,7 +2224,9 @@ SYSCALL_DEFINE3(io_submit, aio_context_t, ctx_id, long, nr,
 		if (ret)
 			break;
 	}
-	blk_finish_plug(&plug);
+
+	if (!(ctx->flags & IOCTX_FLAG_IOPOLL))
+		blk_finish_plug(&plug);
 
 	percpu_ref_put(&ctx->users);
 	return i ? i : ret;
@@ -2068,8 +2351,12 @@ static long do_io_getevents(aio_context_t ctx_id,
 	long ret = -EINVAL;
 
 	if (likely(ioctx)) {
-		if (likely(min_nr <= nr && min_nr >= 0))
-			ret = read_events(ioctx, min_nr, nr, events, until);
+		if (likely(min_nr <= nr && min_nr >= 0)) {
+			if (ioctx->flags & IOCTX_FLAG_IOPOLL)
+				ret = aio_iopoll_check(ioctx, min_nr, nr, events);
+			else
+				ret = read_events(ioctx, min_nr, nr, events, until);
+		}
 		percpu_ref_put(&ioctx->users);
 	}
 
diff --git a/include/uapi/linux/aio_abi.h b/include/uapi/linux/aio_abi.h
index 8387e0af0f76..3b98b5fbacde 100644
--- a/include/uapi/linux/aio_abi.h
+++ b/include/uapi/linux/aio_abi.h
@@ -52,9 +52,11 @@ enum {
  *                   is valid.
  * IOCB_FLAG_IOPRIO - Set if the "aio_reqprio" member of the "struct iocb"
  *                    is valid.
+ * IOCB_FLAG_HIPRI - Use IO completion polling
  */
 #define IOCB_FLAG_RESFD		(1 << 0)
 #define IOCB_FLAG_IOPRIO	(1 << 1)
+#define IOCB_FLAG_HIPRI		(1 << 2)
 
 /* read() from /dev/aio returns these structures. */
 struct io_event {
@@ -106,6 +108,8 @@ struct iocb {
 	__u32	aio_resfd;
 }; /* 64 bytes */
 
+#define IOCTX_FLAG_IOPOLL	(1 << 0)
+
 #undef IFBIG
 #undef IFLITTLE
 
-- 
2.17.1