[PATCHSET v2 RFC 0/11] Add support for async buffered reads

[PATCHSET v2 RFC 0/11] Add support for async buffered reads

[Jens Axboe]

We technically support this already through io_uring, but it's
implemented with a thread backend to support cases where we would
block. This isn't ideal.

After a few prep patches, the core of this patchset is adding support
for async callbacks on page unlock. With this primitive, we can simply
retry the IO operation. With io_uring, this works a lot like poll based
retry for files that support it. If a page is currently locked and
needed, -EIOCBQUEUED is returned with a callback armed. The callers
callback is responsible for restarting the operation.

With this callback primitive, we can add support for
generic_file_buffered_read(), which is what most file systems end up
using for buffered reads. XFS/ext4/btrfs/bdev is wired up, but probably
trivial to add more.

The file flags support for this by setting FMODE_BUF_RASYNC, similar
to what we do for FMODE_NOWAIT. Open to suggestions here if this is
the preferred method or not.

In terms of results, I wrote a small test app that randomly reads 4G
of data in 4K chunks from a file hosted by ext4. The app uses a queue
depth of 32.

preadv for comparison:
	real    1m13.821s
	user    0m0.558s
	sys     0m11.125s
	CPU	~13%

Mainline:
	real    0m12.054s
	user    0m0.111s
	sys     0m5.659s
	CPU	~32% + ~50% == ~82%

This patchset:
	real    0m9.283s
	user    0m0.147s
	sys     0m4.619s
	CPU	~52%
													
The CPU numbers are just a rough estimate. For the mainline io_uring
run, this includes the app itself and all the threads doing IO on its
behalf (32% for the app, ~1.6% per worker and 32 of them). Context
switch rate is much smaller with the patchset, since we only have the
one task performing IO.

The goal here is efficiency. Async thread offload adds latency, and
it also adds noticable overhead on items such as adding pages to the
page cache. By allowing proper async buffered read support, we don't
have X threads hammering on the same inode page cache, we have just
the single app actually doing IO.

Series can also be found here:

https://git.kernel.dk/cgit/linux-block/log/?h=async-buffered.2

or pull from:

git://git.kernel.dk/linux-block async-buffered.2

 fs/block_dev.c            |   2 +-
 fs/btrfs/file.c           |   2 +-
 fs/ext4/file.c            |   2 +-
 fs/io_uring.c             | 102 ++++++++++++++++++++++++++++++++++++++
 fs/xfs/xfs_file.c         |   2 +-
 include/linux/blk_types.h |   3 +-
 include/linux/fs.h        |   5 ++
 include/linux/pagemap.h   |  39 +++++++++++++++
 mm/filemap.c              |  83 +++++++++++++++++++++++++------
 9 files changed, 219 insertions(+), 21 deletions(-)

Changes since v1:

- Fix an issue with inline page locking
- Fix a potential race with __wait_on_page_locked_async()
- Fix a hang related to not setting page_match, thus missing a wakeup

-- 
Jens Axboe

[PATCH 01/11] block: read-ahead submission should imply no-wait as well

[Jens Axboe]

As read-ahead is opportunistic, don't block for request allocation.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/blk_types.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index ccb895f911b1..c296463c15eb 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -374,7 +374,8 @@ enum req_flag_bits {
 #define REQ_INTEGRITY		(1ULL << __REQ_INTEGRITY)
 #define REQ_FUA			(1ULL << __REQ_FUA)
 #define REQ_PREFLUSH		(1ULL << __REQ_PREFLUSH)
-#define REQ_RAHEAD		(1ULL << __REQ_RAHEAD)
+#define REQ_RAHEAD		\
+	((1ULL << __REQ_RAHEAD) | (1ULL << __REQ_NOWAIT))
 #define REQ_BACKGROUND		(1ULL << __REQ_BACKGROUND)
 #define REQ_NOWAIT		(1ULL << __REQ_NOWAIT)
 #define REQ_CGROUP_PUNT		(1ULL << __REQ_CGROUP_PUNT)
-- 
2.26.2

[PATCH 02/11] mm: allow read-ahead with IOCB_NOWAIT set

[Jens Axboe]

The read-ahead shouldn't block, so allow it to be done even if
IOCB_NOWAIT is set in the kiocb.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 mm/filemap.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 23a051a7ef0f..80747f1377d5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2031,8 +2031,6 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 		page = find_get_page(mapping, index);
 		if (!page) {
-			if (iocb->ki_flags & IOCB_NOWAIT)
-				goto would_block;
 			page_cache_sync_readahead(mapping,
 					ra, filp,
 					index, last_index - index);
-- 
2.26.2

[PATCH 03/11] mm: add support for async page locking

[Jens Axboe]

Normally waiting for a page to become unlocked, or locking the page,
requires waiting for IO to complete. Add support for lock_page_async()
and wait_on_page_locked_async(), which are callback based instead. This
allows a caller to get notified when a page becomes unlocked, rather
than wait for it.

We use the iocb->private field to pass in this necessary data for this
to happen. struct wait_page_key is made public, and we define struct
wait_page_async as the interface between the caller and the core.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/fs.h      |  2 ++
 include/linux/pagemap.h | 21 ++++++++++++++++
 mm/filemap.c            | 56 +++++++++++++++++++++++++++++++++++------
 3 files changed, 72 insertions(+), 7 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7e84d823c6a8..82b989695ab9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -314,6 +314,8 @@ enum rw_hint {
 #define IOCB_SYNC		(1 << 5)
 #define IOCB_WRITE		(1 << 6)
 #define IOCB_NOWAIT		(1 << 7)
+/* iocb->private holds wait_page_async struct */
+#define IOCB_WAITQ		(1 << 8)
 
 struct kiocb {
 	struct file		*ki_filp;
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index a8f7bd8ea1c6..e260bcd071e4 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -456,8 +456,21 @@ static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
 	return pgoff;
 }
 
+/* This has the same layout as wait_bit_key - see fs/cachefiles/rdwr.c */
+struct wait_page_key {
+	struct page *page;
+	int bit_nr;
+	int page_match;
+};
+
+struct wait_page_async {
+	struct wait_queue_entry wait;
+	struct wait_page_key key;
+};
+
 extern void __lock_page(struct page *page);
 extern int __lock_page_killable(struct page *page);
+extern int __lock_page_async(struct page *page, struct wait_page_async *wait);
 extern int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
 				unsigned int flags);
 extern void unlock_page(struct page *page);
@@ -494,6 +507,14 @@ static inline int lock_page_killable(struct page *page)
 	return 0;
 }
 
+static inline int lock_page_async(struct page *page,
+				  struct wait_page_async *wait)
+{
+	if (!trylock_page(page))
+		return __lock_page_async(page, wait);
+	return 0;
+}
+
 /*
  * lock_page_or_retry - Lock the page, unless this would block and the
  * caller indicated that it can handle a retry.
diff --git a/mm/filemap.c b/mm/filemap.c
index 80747f1377d5..a01daafd49fd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -990,13 +990,6 @@ void __init pagecache_init(void)
 	page_writeback_init();
 }
 
-/* This has the same layout as wait_bit_key - see fs/cachefiles/rdwr.c */
-struct wait_page_key {
-	struct page *page;
-	int bit_nr;
-	int page_match;
-};
-
 struct wait_page_queue {
 	struct page *page;
 	int bit_nr;
@@ -1210,6 +1203,50 @@ int wait_on_page_bit_killable(struct page *page, int bit_nr)
 }
 EXPORT_SYMBOL(wait_on_page_bit_killable);
 
+static int __wait_on_page_locked_async(struct page *page,
+				       struct wait_page_async *wait, bool set)
+{
+	struct wait_queue_head *q = page_waitqueue(page);
+	int ret = 0;
+
+	wait->key.page = page;
+	wait->key.bit_nr = PG_locked;
+
+	spin_lock_irq(&q->lock);
+	if (set)
+		ret = !trylock_page(page);
+	else
+		ret = PageLocked(page);
+	if (ret) {
+		__add_wait_queue_entry_tail(q, &wait->wait);
+		SetPageWaiters(page);
+		if (set)
+			ret = !trylock_page(page);
+		else
+			ret = PageLocked(page);
+		/*
+		 * If we were succesful now, we know we're still on the
+		 * waitqueue as we're still under the lock. This means it's
+		 * safe to remove and return success, we know the callback
+		 * isn't going to trigger.
+		 */
+		if (!ret)
+			__remove_wait_queue(q, &wait->wait);
+		else
+			ret = -EIOCBQUEUED;
+	}
+	spin_unlock_irq(&q->lock);
+	return ret;
+}
+
+static int wait_on_page_locked_async(struct page *page,
+				     struct wait_page_async *wait)
+{
+	if (!PageLocked(page))
+		return 0;
+	return __wait_on_page_locked_async(compound_head(page), wait, false);
+}
+
 /**
  * put_and_wait_on_page_locked - Drop a reference and wait for it to be unlocked
  * @page: The page to wait for.
@@ -1372,6 +1409,11 @@ int __lock_page_killable(struct page *__page)
 }
 EXPORT_SYMBOL_GPL(__lock_page_killable);
 
+int __lock_page_async(struct page *page, struct wait_page_async *wait)
+{
+	return __wait_on_page_locked_async(page, wait, true);
+}
+
 /*
  * Return values:
  * 1 - page is locked; mmap_sem is still held.
-- 
2.26.2

[PATCH 04/11] mm: support async buffered reads in generic_file_buffered_read()

[Jens Axboe]

Use the async page locking infrastructure, if IOCB_WAITQ is set in the
passed in iocb. The caller must expect an -EIOCBQUEUED return value,
which means that IO is started but not done yet. This is similar to how
O_DIRECT signals the same operation. Once the callback is received by
the caller for IO completion, the caller must retry the operation.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 mm/filemap.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index a01daafd49fd..b49836ff0fdc 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2086,17 +2086,25 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 					index, last_index - index);
 		}
 		if (!PageUptodate(page)) {
-			if (iocb->ki_flags & IOCB_NOWAIT) {
-				put_page(page);
-				goto would_block;
-			}
-
 			/*
 			 * See comment in do_read_cache_page on why
 			 * wait_on_page_locked is used to avoid unnecessarily
 			 * serialisations and why it's safe.
 			 */
-			error = wait_on_page_locked_killable(page);
+			if (iocb->ki_flags & IOCB_WAITQ) {
+				if (written) {
+					put_page(page);
+					goto out;
+				}
+				error = wait_on_page_locked_async(page,
+								iocb->private);
+			} else {
+				if (iocb->ki_flags & IOCB_NOWAIT) {
+					put_page(page);
+					goto would_block;
+				}
+				error = wait_on_page_locked_killable(page);
+			}
 			if (unlikely(error))
 				goto readpage_error;
 			if (PageUptodate(page))
@@ -2184,7 +2192,10 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 
 page_not_up_to_date:
 		/* Get exclusive access to the page ... */
-		error = lock_page_killable(page);
+		if (iocb->ki_flags & IOCB_WAITQ)
+			error = lock_page_async(page, iocb->private);
+		else
+			error = lock_page_killable(page);
 		if (unlikely(error))
 			goto readpage_error;
 
-- 
2.26.2

[PATCH 05/11] fs: add FMODE_BUF_RASYNC

[Jens Axboe]

If set, this indicates that the file system supports IOCB_WAITQ for
buffered reads.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/fs.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 82b989695ab9..0ef5f5973b1c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -175,6 +175,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File does not contribute to nr_files count */
 #define FMODE_NOACCOUNT		((__force fmode_t)0x20000000)
 
+/* File supports async buffered reads */
+#define FMODE_BUF_RASYNC	((__force fmode_t)0x40000000)
+
 /*
  * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
  * that indicates that they should check the contents of the iovec are
-- 
2.26.2

[PATCH 06/11] ext4: flag as supporting buffered async reads

[Jens Axboe]

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/ext4/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0d624250a62b..9f7d9bf427b4 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -826,7 +826,7 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
 			return ret;
 	}
 
-	filp->f_mode |= FMODE_NOWAIT;
+	filp->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC;
 	return dquot_file_open(inode, filp);
 }
 
-- 
2.26.2

[PATCH 07/11] block: flag block devices as supporting IOCB_WAITQ

[Jens Axboe]

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/block_dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 86e2a7134513..ec8dccc81b65 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1851,7 +1851,7 @@ static int blkdev_open(struct inode * inode, struct file * filp)
 	 */
 	filp->f_flags |= O_LARGEFILE;
 
-	filp->f_mode |= FMODE_NOWAIT;
+	filp->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC;
 
 	if (filp->f_flags & O_NDELAY)
 		filp->f_mode |= FMODE_NDELAY;
-- 
2.26.2

[PATCH 08/11] xfs: flag files as supporting buffered async reads

[Jens Axboe]

XFS uses generic_file_read_iter(), which already supports this.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/xfs/xfs_file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 4b8bdecc3863..97f44fbf17f2 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1080,7 +1080,7 @@ xfs_file_open(
 		return -EFBIG;
 	if (XFS_FORCED_SHUTDOWN(XFS_M(inode->i_sb)))
 		return -EIO;
-	file->f_mode |= FMODE_NOWAIT;
+	file->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC;
 	return 0;
 }
 
-- 
2.26.2

[PATCH 09/11] btrfs: flag files as supporting buffered async reads

[Jens Axboe]

btrfs uses generic_file_read_iter(), which already supports this.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/btrfs/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 719e68ab552c..c933b6a1b4a8 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3480,7 +3480,7 @@ static loff_t btrfs_file_llseek(struct file *file, loff_t offset, int whence)
 
 static int btrfs_file_open(struct inode *inode, struct file *filp)
 {
-	filp->f_mode |= FMODE_NOWAIT;
+	filp->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC;
 	return generic_file_open(inode, filp);
 }
 
-- 
2.26.2

[PATCH 10/11] mm: add kiocb_wait_page_async_init() helper

[Jens Axboe]

Checks if the file supports it, and initializes the values that we need.
Caller passes in 'data' pointer, if any, and the callback function to
be used.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/pagemap.h | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e260bcd071e4..21ced353310a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -468,6 +468,24 @@ struct wait_page_async {
 	struct wait_page_key key;
 };
 
+static inline int kiocb_wait_page_async_init(struct kiocb *kiocb,
+					     struct wait_page_async *wait,
+					     wait_queue_func_t func,
+					     void *data)
+{
+	if (kiocb->ki_filp->f_mode & FMODE_BUF_RASYNC) {
+		wait->wait.func = func;
+		wait->wait.private = data;
+		wait->wait.flags = 0;
+		INIT_LIST_HEAD(&wait->wait.entry);
+		kiocb->ki_flags |= IOCB_WAITQ;
+		kiocb->private = wait;
+		return 0;
+	}
+
+	return -EOPNOTSUPP;
+}
+
 extern void __lock_page(struct page *page);
 extern int __lock_page_killable(struct page *page);
 extern int __lock_page_async(struct page *page, struct wait_page_async *wait);
-- 
2.26.2

[PATCH 11/11] io_uring: support true async buffered reads, if file provides it

[Jens Axboe]

If the file is flagged with FMODE_BUF_RASYNC, then we don't have to punt
the buffered read to an io-wq worker. Instead we can rely on page
unlocking callbacks to support retry based async IO. This is a lot more
efficient than doing async thread offload.

The retry is done similarly to how we handle poll based retry. From
the unlock callback, we simply queue the retry to a task_work based
handler.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io_uring.c | 102 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index e95481c552ff..9eeae10db648 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -498,6 +498,8 @@ struct io_async_rw {
 	struct iovec			*iov;
 	ssize_t				nr_segs;
 	ssize_t				size;
+	struct wait_page_async		wait;
+	struct callback_head		task_work;
 };
 
 struct io_async_ctx {
@@ -2568,6 +2570,102 @@ static int io_read_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe,
 	return 0;
 }
 
+static void io_async_buf_cancel(struct callback_head *cb)
+{
+	struct io_async_rw *rw;
+	struct io_ring_ctx *ctx;
+	struct io_kiocb *req;
+
+	rw = container_of(cb, struct io_async_rw, task_work);
+	req = rw->wait.wait.private;
+	ctx = req->ctx;
+
+	spin_lock_irq(&ctx->completion_lock);
+	io_cqring_fill_event(req, -ECANCELED);
+	io_commit_cqring(ctx);
+	spin_unlock_irq(&ctx->completion_lock);
+
+	io_cqring_ev_posted(ctx);
+	req_set_fail_links(req);
+	io_double_put_req(req);
+}
+
+static void io_async_buf_retry(struct callback_head *cb)
+{
+	struct io_async_rw *rw;
+	struct io_ring_ctx *ctx;
+	struct io_kiocb *req;
+
+	rw = container_of(cb, struct io_async_rw, task_work);
+	req = rw->wait.wait.private;
+	ctx = req->ctx;
+
+	__set_current_state(TASK_RUNNING);
+	mutex_lock(&ctx->uring_lock);
+	__io_queue_sqe(req, NULL);
+	mutex_unlock(&ctx->uring_lock);
+}
+
+static int io_async_buf_func(struct wait_queue_entry *wait, unsigned mode,
+			     int sync, void *arg)
+{
+	struct wait_page_async *wp;
+	struct io_kiocb *req = wait->private;
+	struct io_async_rw *rw = &req->io->rw;
+	struct wait_page_key *key = arg;
+	struct task_struct *tsk;
+	int ret;
+
+	wp = container_of(wait, struct wait_page_async, wait);
+	if (wp->key.page != key->page)
+		return 0;
+	key->page_match = 1;
+	if (wp->key.bit_nr != key->bit_nr)
+		return 0;
+	if (test_bit(PG_locked, &key->page->flags))
+		return -1;
+
+	list_del_init(&wait->entry);
+
+	init_task_work(&rw->task_work, io_async_buf_retry);
+	/* submit ref gets dropped, acquire a new one */
+	refcount_inc(&req->refs);
+	tsk = req->task;
+	ret = task_work_add(tsk, &rw->task_work, true);
+	if (unlikely(ret)) {
+		/* queue just for cancelation */
+		init_task_work(&rw->task_work, io_async_buf_cancel);
+		tsk = io_wq_get_task(req->ctx->io_wq);
+		task_work_add(tsk, &rw->task_work, true);
+	}
+	wake_up_process(tsk);
+	return 1;
+}
+
+static bool io_rw_should_retry(struct io_kiocb *req)
+{
+	struct kiocb *kiocb = &req->rw.kiocb;
+	int ret;
+
+	/* already tried, or we're doing O_DIRECT */
+	if (kiocb->ki_flags & (IOCB_DIRECT | IOCB_WAITQ))
+		return false;
+	/*
+	 * just use poll if we can, and don't attempt if the fs doesn't
+	 * support callback based unlocks
+	 */
+	if (file_can_poll(req->file) || !(req->file->f_mode & FMODE_BUF_RASYNC))
+		return false;
+
+	ret = kiocb_wait_page_async_init(kiocb, &req->io->rw.wait,
+						io_async_buf_func, req);
+	if (ret)
+		return false;
+	get_task_struct(current);
+	req->task = current;
+	return true;
+}
+
 static int io_read(struct io_kiocb *req, bool force_nonblock)
 {
 	struct iovec inline_vecs[UIO_FASTIOV], *iovec = inline_vecs;
@@ -2601,6 +2699,7 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
 	if (!ret) {
 		ssize_t ret2;
 
+retry:
 		if (req->file->f_op->read_iter)
 			ret2 = call_read_iter(req->file, kiocb, &iter);
 		else
@@ -2619,6 +2718,9 @@ static int io_read(struct io_kiocb *req, bool force_nonblock)
 			if (!(req->flags & REQ_F_NOWAIT) &&
 			    !file_can_poll(req->file))
 				req->flags |= REQ_F_MUST_PUNT;
+			if (io_rw_should_retry(req))
+				goto retry;
+			kiocb->ki_flags &= ~IOCB_WAITQ;
 			return -EAGAIN;
 		}
 	}
-- 
2.26.2

[PATCHSET RFC 0/11] Add support for async buffered reads

[Jens Axboe]

We technically support this already through io_uring, but it's
implemented with a thread backend to support cases where we would
block. This isn't ideal.

After a few prep patches, the core of this patchset is adding support
for async callbacks on page unlock. With this primitive, we can simply
retry the IO operation. With io_uring, this works a lot like poll based
retry for files that support it. If a page is currently locked and
needed, -EIOCBQUEUED is returned with a callback armed. The callers
callback is responsible for restarting the operation.

With this callback primitive, we can add support for
generic_file_buffered_read(), which is what most file systems end up
using for buffered reads. XFS/ext4/btrfs/bdev is wired up, but probably
trivial to add more.

The file flags support for this by setting FMODE_BUF_RASYNC, similar
to what we do for FMODE_NOWAIT. Open to suggestions here if this is
the preferred method or not.

In terms of results, I wrote a small test app that randomly reads 4G
of data in 4K chunks from a file hosted by ext4. The app uses a queue
depth of 32.

preadv for comparison:
	real    1m13.821s
	user    0m0.558s
	sys     0m11.125s
	CPU	~13%

Mainline:
	real    0m12.054s
	user    0m0.111s
	sys     0m5.659s
	CPU	~32% + ~50% == ~82%

This patchset:
	real    0m9.283s
	user    0m0.147s
	sys     0m4.619s
	CPU	~52%
	
The CPU numbers are just a rough estimate. For the mainline io_uring
run, this includes the app itself and all the threads doing IO on its
behalf (32% for the app, ~1.6% per worker and 32 of them). Context
switch rate is much smaller with the patchset, since we only have the
one task performing IO.

The goal here is efficiency. Async thread offload adds latency, and
it also adds noticable overhead on items such as adding pages to the
page cache. By allowing proper async buffered read support, we don't
have X threads hammering on the same inode page cache, we have just
the single app actually doing IO.

Series can also be found here:

https://git.kernel.dk/cgit/linux-block/log/?h=async-buffered

or pull from:

git://git.kernel.dk/linux-block async-buffered

 fs/block_dev.c            |   2 +-
 fs/btrfs/file.c           |   2 +-
 fs/ext4/file.c            |   2 +-
 fs/io_uring.c             | 101 ++++++++++++++++++++++++++++++++++++++
 fs/xfs/xfs_file.c         |   2 +-
 include/linux/blk_types.h |   3 +-
 include/linux/fs.h        |   5 ++
 include/linux/pagemap.h   |  40 +++++++++++++++
 mm/filemap.c              |  71 +++++++++++++++++++++------
 9 files changed, 207 insertions(+), 21 deletions(-)

-- 
Jens Axboe

[PATCH 01/11] block: read-ahead submission should imply no-wait as well

[Jens Axboe]

As read-ahead is opportunistic, don't block for request allocation.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/blk_types.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index ccb895f911b1..c296463c15eb 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -374,7 +374,8 @@ enum req_flag_bits {
 #define REQ_INTEGRITY		(1ULL << __REQ_INTEGRITY)
 #define REQ_FUA			(1ULL << __REQ_FUA)
 #define REQ_PREFLUSH		(1ULL << __REQ_PREFLUSH)
-#define REQ_RAHEAD		(1ULL << __REQ_RAHEAD)
+#define REQ_RAHEAD		\
+	((1ULL << __REQ_RAHEAD) | (1ULL << __REQ_NOWAIT))
 #define REQ_BACKGROUND		(1ULL << __REQ_BACKGROUND)
 #define REQ_NOWAIT		(1ULL << __REQ_NOWAIT)
 #define REQ_CGROUP_PUNT		(1ULL << __REQ_CGROUP_PUNT)
-- 
2.26.2