[PATCH 0/8 v7] No wait AIO

[PATCH 0/8 v7] No wait AIO

[Goldwyn Rodrigues]

Formerly known as non-blocking AIO.

This series adds nonblocking feature to asynchronous I/O writes.
io_submit() can be delayed because of a number of reason:
 - Block allocation for files
 - Data writebacks for direct I/O
 - Sleeping because of waiting to acquire i_rwsem
 - Congested block device

The goal of the patch series is to return -EAGAIN/-EWOULDBLOCK if
any of these conditions are met. This way userspace can push most
of the write()s to the kernel to the best of its ability to complete
and if it returns -EAGAIN, can defer it to another thread.

In order to enable this, IOCB_RW_FLAG_NOWAIT is introduced in
uapi/linux/aio_abi.h. If set for aio_rw_flags, it translates to
IOCB_NOWAIT for struct iocb, REQ_NOWAIT for bio.bi_opf and IOMAP_NOWAIT for
iomap. aio_rw_flags is a new flag replacing aio_reserved1. We could
not use aio_flags because it is not currently checked for invalidity
in the kernel.

This feature is provided for direct I/O of asynchronous I/O only. I have
tested it against xfs, ext4, and btrfs while I intend to add more filesystems.
The nowait feature is for request based devices. In the future, I intend to
add support to stacked devices such as md.

Applications will have to check supportability
by sending a async direct write and any other error besides -EAGAIN
would mean it is not supported.

Changes since v1:
 + changed name from _NONBLOCKING to *_NOWAIT
 + filemap_range_has_page call moved to closer to (just before) calling filemap_write_and_wait_range().
 + BIO_NOWAIT limited to get_request()
 + XFS fixes 
	- included reflink 
	- use of xfs_ilock_nowait() instead of a XFS_IOLOCK_NONBLOCKING flag
	- Translate the flag through IOMAP_NOWAIT (iomap) to check for
	  block allocation for the file.
 + ext4 coding style

Changes since v2:
 + Using aio_reserved1 as aio_rw_flags instead of aio_flags
 + blk-mq support
 + xfs uptodate with kernel and reflink changes

 Changes since v3:
  + Added FS_NOWAIT, which is set if the filesystem supports NOWAIT feature.
  + Checks in generic_make_request() to make sure BIO_NOWAIT comes in
    for async direct writes only.
  + Added QUEUE_FLAG_NOWAIT, which is set if the device supports BIO_NOWAIT.
    This is added (rather not set) to block devices such as dm/md currently.

 Changes since v4:
  + Ported AIO code to use RWF_* flags. Check for RWF_* flags in
    generic_file_write_iter().
  + Changed IOCB_RW_FLAGS_NOWAIT to RWF_NOWAIT.

 Changes since v5:
  + BIO_NOWAIT to REQ_NOWAIT
  + Common helper for RWF flags.

 Changes since v6:
  + REQ_NOWAIT will be ignored for request based devices since they
    cannot block. So, removed QUEUE_FLAG_NOWAIT since it is not
    required in the current implementation. It will be resurrected
    when we program for stacked devices.
  + changed kiocb_rw_flags() to kiocb_set_rw_flags() in order to accomodate
    for errors. Moved checks in the function.

-- 
Goldwyn

[PATCH 1/8] Use RWF_* flags for AIO operations

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

RWF_* flags is used for preadv2/pwritev2 calls. Port to use
it for aio operations as well. For this, aio_rw_flags is
introduced in struct iocb (using aio_reserved1) which will
carry these flags.

This is a precursor to the nowait AIO calls.

Note, the only place RWF_HIPRI comes in effect is dio_await_one().
All the rest of the locations, aio code return -EIOCBQUEUED before the
checks for RWF_HIPRI.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/aio.c                     |  8 +++++++-
 fs/read_write.c              | 12 +++---------
 include/linux/fs.h           | 14 ++++++++++++++
 include/uapi/linux/aio_abi.h |  2 +-
 4 files changed, 25 insertions(+), 11 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index f52d925ee259..020fa0045e3c 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1541,7 +1541,7 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 	ssize_t ret;
 
 	/* enforce forwards compatibility on users */
-	if (unlikely(iocb->aio_reserved1 || iocb->aio_reserved2)) {
+	if (unlikely(iocb->aio_reserved2)) {
 		pr_debug("EINVAL: reserve field set\n");
 		return -EINVAL;
 	}
@@ -1586,6 +1586,12 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 		req->common.ki_flags |= IOCB_EVENTFD;
 	}
 
+	ret = kiocb_set_rw_flags(&req->common, iocb->aio_rw_flags);
+	if (unlikely(ret)) {
+		pr_debug("EINVAL: aio_rw_flags\n");
+		goto out_put_req;
+	}
+
 	ret = put_user(KIOCB_KEY, &user_iocb->aio_key);
 	if (unlikely(ret)) {
 		pr_debug("EFAULT: aio_key\n");
diff --git a/fs/read_write.c b/fs/read_write.c
index c4f88afbc67f..362f91cd8d66 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -678,16 +678,10 @@ static ssize_t do_iter_readv_writev(struct file *filp, struct iov_iter *iter,
 	struct kiocb kiocb;
 	ssize_t ret;
 
-	if (flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC))
-		return -EOPNOTSUPP;
-
 	init_sync_kiocb(&kiocb, filp);
-	if (flags & RWF_HIPRI)
-		kiocb.ki_flags |= IOCB_HIPRI;
-	if (flags & RWF_DSYNC)
-		kiocb.ki_flags |= IOCB_DSYNC;
-	if (flags & RWF_SYNC)
-		kiocb.ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
+	ret = kiocb_set_rw_flags(&kiocb, flags);
+	if (ret)
+		return ret;
 	kiocb.ki_pos = *ppos;
 
 	if (type == READ)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7251f7bb45e8..869c9a6fe58d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3049,6 +3049,20 @@ static inline int iocb_flags(struct file *file)
 	return res;
 }
 
+static inline int kiocb_set_rw_flags(struct kiocb *ki, int flags)
+{
+	if (unlikely(flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC)))
+		return -EOPNOTSUPP;
+
+	if (flags & RWF_HIPRI)
+		ki->ki_flags |= IOCB_HIPRI;
+	if (flags & RWF_DSYNC)
+		ki->ki_flags |= IOCB_DSYNC;
+	if (flags & RWF_SYNC)
+		ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
+	return 0;
+}
+
 static inline ino_t parent_ino(struct dentry *dentry)
 {
 	ino_t res;
diff --git a/include/uapi/linux/aio_abi.h b/include/uapi/linux/aio_abi.h
index bb2554f7fbd1..a2d4a8ac94ca 100644
--- a/include/uapi/linux/aio_abi.h
+++ b/include/uapi/linux/aio_abi.h
@@ -79,7 +79,7 @@ struct io_event {
 struct iocb {
 	/* these are internal to the kernel/libc. */
 	__u64	aio_data;	/* data to be returned in event's data */
-	__u32	PADDED(aio_key, aio_reserved1);
+	__u32	PADDED(aio_key, aio_rw_flags);
 				/* the kernel sets aio_key to the req # */
 
 	/* common fields */
-- 
2.12.0

[PATCH 2/8] nowait aio: Introduce RWF_NOWAIT

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

This flag informs kernel to bail out if an AIO request will block
for reasons such as file allocations, or a writeback triggered,
or would block while allocating requests while performing
direct I/O.

Unfortunately, aio_flags is not checked for validity, which would
break existing applications which have it set to anything besides zero
or IOCB_FLAG_RESFD. So, we are using aio_reserved1 and renaming it
to aio_rw_flags.

RWF_NOWAIT is translated to IOCB_NOWAIT for iocb->ki_flags.

The check for -EOPNOTSUPP is placed in generic_file_write_iter(). This
is called by most filesystems, either through fsops.write_iter() or through
the function defined by write_iter(). If not, we perform the check defined
by .write_iter() which is called for direct IO specifically.

Filesystems xfs, btrfs and ext4 would be supported in the following patches.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/9p/vfs_file.c        | 3 +++
 fs/aio.c                | 6 ++++++
 fs/ceph/file.c          | 3 +++
 fs/cifs/file.c          | 3 +++
 fs/fuse/file.c          | 3 +++
 fs/nfs/direct.c         | 3 +++
 fs/ocfs2/file.c         | 3 +++
 include/linux/fs.h      | 5 ++++-
 include/uapi/linux/fs.h | 1 +
 mm/filemap.c            | 3 +++
 10 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/fs/9p/vfs_file.c b/fs/9p/vfs_file.c
index 3de3b4a89d89..403681db7723 100644
--- a/fs/9p/vfs_file.c
+++ b/fs/9p/vfs_file.c
@@ -411,6 +411,9 @@ v9fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	loff_t origin;
 	int err = 0;
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	retval = generic_write_checks(iocb, from);
 	if (retval <= 0)
 		return retval;
diff --git a/fs/aio.c b/fs/aio.c
index 020fa0045e3c..ea9f8581d902 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1592,6 +1592,12 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
 		goto out_put_req;
 	}
 
+	if ((req->common.ki_flags & IOCB_NOWAIT) &&
+		!(req->common.ki_flags & IOCB_DIRECT)) {
+		ret = -EOPNOTSUPP;
+		goto out_put_req;
+	}
+
 	ret = put_user(KIOCB_KEY, &user_iocb->aio_key);
 	if (unlikely(ret)) {
 		pr_debug("EFAULT: aio_key\n");
diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 26cc95421cca..af28419b1731 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -1267,6 +1267,9 @@ static ssize_t ceph_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	int err, want, got;
 	loff_t pos;
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	if (ceph_snap(inode) != CEPH_NOSNAP)
 		return -EROFS;
 
diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 21d404535739..f8858a06e119 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -2638,6 +2638,9 @@ ssize_t cifs_user_writev(struct kiocb *iocb, struct iov_iter *from)
 	 * write request.
 	 */
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	rc = generic_write_checks(iocb, from);
 	if (rc <= 0)
 		return rc;
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index ec238fb5a584..72786e798319 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -1425,6 +1425,9 @@ static ssize_t fuse_direct_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct fuse_io_priv io = FUSE_IO_PRIV_SYNC(file);
 	ssize_t res;
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	if (is_bad_inode(inode))
 		return -EIO;
 
diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index c1b5fed7c863..dcea0caa5cb5 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -996,6 +996,9 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter)
 	dfprintk(FILE, "NFS: direct write(%pD2, %zd@%Ld)\n",
 		file, iov_iter_count(iter), (long long) iocb->ki_pos);
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	result = generic_write_checks(iocb, iter);
 	if (result <= 0)
 		return result;
diff --git a/fs/ocfs2/file.c b/fs/ocfs2/file.c
index bfeb647459d9..e7f8ba890305 100644
--- a/fs/ocfs2/file.c
+++ b/fs/ocfs2/file.c
@@ -2235,6 +2235,9 @@ static ssize_t ocfs2_file_write_iter(struct kiocb *iocb,
 	if (count == 0)
 		return 0;
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	direct_io = iocb->ki_flags & IOCB_DIRECT ? 1 : 0;
 
 	inode_lock(inode);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 869c9a6fe58d..4cb62e032b70 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -270,6 +270,7 @@ struct writeback_control;
 #define IOCB_DSYNC		(1 << 4)
 #define IOCB_SYNC		(1 << 5)
 #define IOCB_WRITE		(1 << 6)
+#define IOCB_NOWAIT		(1 << 7)
 
 struct kiocb {
 	struct file		*ki_filp;
@@ -3051,7 +3052,7 @@ static inline int iocb_flags(struct file *file)
 
 static inline int kiocb_set_rw_flags(struct kiocb *ki, int flags)
 {
-	if (unlikely(flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC)))
+	if (unlikely(flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT)))
 		return -EOPNOTSUPP;
 
 	if (flags & RWF_HIPRI)
@@ -3060,6 +3061,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, int flags)
 		ki->ki_flags |= IOCB_DSYNC;
 	if (flags & RWF_SYNC)
 		ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
+	if (flags & RWF_NOWAIT)
+		ki->ki_flags |= IOCB_NOWAIT;
 	return 0;
 }
 
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 048a85e9f017..7bcaef101876 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -347,5 +347,6 @@ struct fscrypt_policy {
 #define RWF_HIPRI			0x00000001 /* high priority request, poll if possible */
 #define RWF_DSYNC			0x00000002 /* per-IO O_DSYNC */
 #define RWF_SYNC			0x00000004 /* per-IO O_SYNC */
+#define RWF_NOWAIT			0x00000008 /* per-IO, return -EAGAIN if operation would block */
 
 #endif /* _UAPI_LINUX_FS_H */
diff --git a/mm/filemap.c b/mm/filemap.c
index 1694623a6289..d51670b7fe6b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2982,6 +2982,9 @@ ssize_t generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct inode *inode = file->f_mapping->host;
 	ssize_t ret;
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 	inode_lock(inode);
 	ret = generic_write_checks(iocb, from);
 	if (ret > 0)
-- 
2.12.0

[PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Find out if the write will trigger a wait due to writeback. If yes,
return -EAGAIN.

This introduces a new function filemap_range_has_page() which
returns true if the file's mapping has a page within the range
mentioned.

Return -EINVAL for buffered AIO: there are multiple causes of
delay such as page locks, dirty throttling logic, page loading
from disk etc. which cannot be taken care of.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 include/linux/fs.h |  2 ++
 mm/filemap.c       | 50 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4cb62e032b70..24d5c123788f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2514,6 +2514,8 @@ extern int filemap_fdatawait(struct address_space *);
 extern void filemap_fdatawait_keep_errors(struct address_space *);
 extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
 				   loff_t lend);
+extern int filemap_range_has_page(struct address_space *, loff_t lstart,
+				   loff_t lend);
 extern int filemap_write_and_wait(struct address_space *mapping);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);
diff --git a/mm/filemap.c b/mm/filemap.c
index d51670b7fe6b..48b83d1d4a30 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -376,6 +376,39 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);
 
+/**
+ * filemap_range_has_page - check if a page exists in range.
+ * @mapping:		address space structure to wait for
+ * @start_byte:		offset in bytes where the range starts
+ * @end_byte:		offset in bytes where the range ends (inclusive)
+ *
+ * Find at least one page in the range supplied, usually used to check if
+ * direct writing in this range will trigger a writeback.
+ */
+int filemap_range_has_page(struct address_space *mapping,
+				loff_t start_byte, loff_t end_byte)
+{
+	pgoff_t index = start_byte >> PAGE_SHIFT;
+	pgoff_t end = end_byte >> PAGE_SHIFT;
+	struct pagevec pvec;
+	int ret;
+
+	if (end_byte < start_byte)
+		return 0;
+
+	if (mapping->nrpages == 0)
+		return 0;
+
+	pagevec_init(&pvec, 0);
+	ret = pagevec_lookup(&pvec, mapping, index, 1);
+	if (!ret)
+		return 0;
+	ret = (pvec.pages[0]->index <= end);
+	pagevec_release(&pvec);
+	return ret;
+}
+EXPORT_SYMBOL(filemap_range_has_page);
+
 static int __filemap_fdatawait_range(struct address_space *mapping,
 				     loff_t start_byte, loff_t end_byte)
 {
@@ -2640,6 +2673,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 
 	pos = iocb->ki_pos;
 
+	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
+		return -EINVAL;
+
 	if (limit != RLIM_INFINITY) {
 		if (iocb->ki_pos >= limit) {
 			send_sig(SIGXFSZ, current, 0);
@@ -2709,9 +2745,17 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	write_len = iov_iter_count(from);
 	end = (pos + write_len - 1) >> PAGE_SHIFT;
 
-	written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
-	if (written)
-		goto out;
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		/* If there are pages to writeback, return */
+		if (filemap_range_has_page(inode->i_mapping, pos,
+					   pos + iov_iter_count(from)))
+			return -EAGAIN;
+	} else {
+		written = filemap_write_and_wait_range(mapping, pos,
+							pos + write_len - 1);
+		if (written)
+			goto out;
+	}
 
 	/*
 	 * After a write we want buffered reads to be sure to go to disk to get
-- 
2.12.0

[PATCH 4/8] nowait-aio: Introduce IOMAP_NOWAIT

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

IOCB_NOWAIT translates to IOMAP_NOWAIT for iomaps.
This is used by XFS in the XFS patch.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/iomap.c            | 2 ++
 include/linux/iomap.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index 141c3cd55a8b..d1c81753d411 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -885,6 +885,8 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
 	} else {
 		dio->flags |= IOMAP_DIO_WRITE;
 		flags |= IOMAP_WRITE;
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			flags |= IOMAP_NOWAIT;
 	}
 
 	if (mapping->nrpages) {
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 7291810067eb..53f6af89c625 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -51,6 +51,7 @@ struct iomap {
 #define IOMAP_REPORT		(1 << 2) /* report extent status, e.g. FIEMAP */
 #define IOMAP_FAULT		(1 << 3) /* mapping for page fault */
 #define IOMAP_DIRECT		(1 << 4) /* direct I/O */
+#define IOMAP_NOWAIT		(1 << 5) /* Don't wait for writeback */
 
 struct iomap_ops {
 	/*
-- 
2.12.0

[PATCH 5/8] nowait aio: return on congested block device

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

A new bio operation flag REQ_NOWAIT is introduced to identify bio's
orignating from iocb with IOCB_NOWAIT. This flag indicates
to return immediately if a request cannot be made instead
of retrying.

Stacked devices such as md (the ones with make_request_fn hooks)
currently are not supported because it may block for housekeeping.
For example, an md can have a part of the device suspended.
For this reason, only request based devices are supported.
In the future, this feature will be expanded to stacked devices
by teaching them how to handle the REQ_NOWAIT flags.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 block/blk-core.c          | 24 ++++++++++++++++++++++--
 block/blk-mq-sched.c      |  3 +++
 block/blk-mq.c            |  4 ++++
 fs/direct-io.c            | 10 ++++++++--
 include/linux/bio.h       |  6 ++++++
 include/linux/blk_types.h |  2 ++
 6 files changed, 45 insertions(+), 4 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index d772c221cc17..effe934b806b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1232,6 +1232,11 @@ static struct request *get_request(struct request_queue *q, unsigned int op,
 	if (!IS_ERR(rq))
 		return rq;
 
+	if (op & REQ_NOWAIT) {
+		blk_put_rl(rl);
+		return ERR_PTR(-EAGAIN);
+	}
+
 	if (!gfpflags_allow_blocking(gfp_mask) || unlikely(blk_queue_dying(q))) {
 		blk_put_rl(rl);
 		return rq;
@@ -1870,6 +1875,17 @@ generic_make_request_checks(struct bio *bio)
 		goto end_io;
 	}
 
+	/*
+	 * For a REQ_NOWAIT based request, return -EOPNOTSUPP
+	 * if queue does not have QUEUE_FLAG_NOWAIT_SUPPORT set
+	 * and if it is not a request based queue.
+	 */
+
+	if ((bio->bi_opf & REQ_NOWAIT) && !queue_is_rq_based(q)) {
+		err = -EOPNOTSUPP;
+		goto end_io;
+	}
+
 	part = bio->bi_bdev->bd_part;
 	if (should_fail_request(part, bio->bi_iter.bi_size) ||
 	    should_fail_request(&part_to_disk(part)->part0,
@@ -2021,7 +2037,7 @@ blk_qc_t generic_make_request(struct bio *bio)
 	do {
 		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
 
-		if (likely(blk_queue_enter(q, false) == 0)) {
+		if (likely(blk_queue_enter(q, bio->bi_opf & REQ_NOWAIT) == 0)) {
 			struct bio_list lower, same;
 
 			/* Create a fresh bio_list for all subordinate requests */
@@ -2046,7 +2062,11 @@ blk_qc_t generic_make_request(struct bio *bio)
 			bio_list_merge(&bio_list_on_stack[0], &same);
 			bio_list_merge(&bio_list_on_stack[0], &bio_list_on_stack[1]);
 		} else {
-			bio_io_error(bio);
+			if (unlikely(!blk_queue_dying(q) &&
+					(bio->bi_opf & REQ_NOWAIT)))
+				bio_wouldblock_error(bio);
+			else
+				bio_io_error(bio);
 		}
 		bio = bio_list_pop(&bio_list_on_stack[0]);
 	} while (bio);
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index c974a1bbf4cb..019d881d62b7 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -119,6 +119,9 @@ struct request *blk_mq_sched_get_request(struct request_queue *q,
 	if (likely(!data->hctx))
 		data->hctx = blk_mq_map_queue(q, data->ctx->cpu);
 
+	if (op & REQ_NOWAIT)
+		data->flags |= BLK_MQ_REQ_NOWAIT;
+
 	if (e) {
 		data->flags |= BLK_MQ_REQ_INTERNAL;
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index c7836a1ded97..d7613ae6a269 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1538,6 +1538,8 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 	rq = blk_mq_sched_get_request(q, bio, bio->bi_opf, &data);
 	if (unlikely(!rq)) {
 		__wbt_done(q->rq_wb, wb_acct);
+		if (bio->bi_opf & REQ_NOWAIT)
+			bio_wouldblock_error(bio);
 		return BLK_QC_T_NONE;
 	}
 
@@ -1662,6 +1664,8 @@ static blk_qc_t blk_sq_make_request(struct request_queue *q, struct bio *bio)
 	rq = blk_mq_sched_get_request(q, bio, bio->bi_opf, &data);
 	if (unlikely(!rq)) {
 		__wbt_done(q->rq_wb, wb_acct);
+		if (bio->bi_opf & REQ_NOWAIT)
+			bio_wouldblock_error(bio);
 		return BLK_QC_T_NONE;
 	}
 
diff --git a/fs/direct-io.c b/fs/direct-io.c
index a04ebea77de8..139ebd5ae1c7 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -480,8 +480,12 @@ static int dio_bio_complete(struct dio *dio, struct bio *bio)
 	unsigned i;
 	int err;
 
-	if (bio->bi_error)
-		dio->io_error = -EIO;
+	if (bio->bi_error) {
+		if (bio->bi_error == -EAGAIN && (bio->bi_opf & REQ_NOWAIT))
+			dio->io_error = -EAGAIN;
+		else
+			dio->io_error = -EIO;
+	}
 
 	if (dio->is_async && dio->op == REQ_OP_READ && dio->should_dirty) {
 		err = bio->bi_error;
@@ -1197,6 +1201,8 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
 	if (iov_iter_rw(iter) == WRITE) {
 		dio->op = REQ_OP_WRITE;
 		dio->op_flags = REQ_SYNC | REQ_IDLE;
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			dio->op_flags |= REQ_NOWAIT;
 	} else {
 		dio->op = REQ_OP_READ;
 	}
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 8e521194f6fc..1a9270744b1e 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -425,6 +425,12 @@ static inline void bio_io_error(struct bio *bio)
 	bio_endio(bio);
 }
 
+static inline void bio_wouldblock_error(struct bio *bio)
+{
+	bio->bi_error = -EAGAIN;
+	bio_endio(bio);
+}
+
 struct request_queue;
 extern int bio_phys_segments(struct request_queue *, struct bio *);
 
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index d703acb55d0f..5ce4da30ba43 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -187,6 +187,7 @@ enum req_flag_bits {
 	__REQ_PREFLUSH,		/* request for cache flush */
 	__REQ_RAHEAD,		/* read ahead, can fail anytime */
 	__REQ_BACKGROUND,	/* background IO */
+	__REQ_NOWAIT,		/* Don't wait if request will block */
 	__REQ_NR_BITS,		/* stops here */
 };
 
@@ -203,6 +204,7 @@ enum req_flag_bits {
 #define REQ_PREFLUSH		(1ULL << __REQ_PREFLUSH)
 #define REQ_RAHEAD		(1ULL << __REQ_RAHEAD)
 #define REQ_BACKGROUND		(1ULL << __REQ_BACKGROUND)
+#define REQ_NOWAIT		(1ULL << __REQ_NOWAIT)
 
 #define REQ_FAILFAST_MASK \
 	(REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT | REQ_FAILFAST_DRIVER)
-- 
2.12.0

[PATCH 6/8] nowait aio: ext4

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Return EAGAIN if any of the following checks fail for direct I/O:
  + i_rwsem is lockable
  + Writing beyond end of file (will trigger allocation)
  + Blocks are not allocated at the write location

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 fs/ext4/file.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index cefa9835f275..2efdc6d4d3e8 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -216,7 +216,13 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		return ext4_dax_write_iter(iocb, from);
 #endif
 
-	inode_lock(inode);
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		if (!inode_trylock(inode))
+			return -EAGAIN;
+	} else {
+		inode_lock(inode);
+	}
+
 	ret = ext4_write_checks(iocb, from);
 	if (ret <= 0)
 		goto out;
@@ -235,9 +241,15 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 
 	iocb->private = &overwrite;
 	/* Check whether we do a DIO overwrite or not */
-	if (o_direct && ext4_should_dioread_nolock(inode) && !unaligned_aio &&
-	    ext4_overwrite_io(inode, iocb->ki_pos, iov_iter_count(from)))
-		overwrite = 1;
+	if (o_direct && !unaligned_aio) {
+		if (ext4_overwrite_io(inode, iocb->ki_pos, iov_iter_count(from))) {
+			if (ext4_should_dioread_nolock(inode))
+				overwrite = 1;
+		} else if (iocb->ki_flags & IOCB_NOWAIT) {
+			ret = -EAGAIN;
+			goto out;
+		}
+	}
 
 	ret = __generic_file_write_iter(iocb, from);
 	inode_unlock(inode);
-- 
2.12.0

[PATCH 7/8] nowait aio: xfs

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

If IOCB_NOWAIT is set, bail if the i_rwsem is not lockable
immediately.

IF IOMAP_NOWAIT is set, return EAGAIN in xfs_file_iomap_begin
if it needs allocation either due to file extension, writing to a hole,
or COW or waiting for other DIOs to finish.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
 fs/xfs/xfs_file.c  | 19 ++++++++++++++-----
 fs/xfs/xfs_iomap.c | 17 +++++++++++++++++
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 35703a801372..b307940e7d56 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -541,8 +541,11 @@ xfs_file_dio_aio_write(
 		iolock = XFS_IOLOCK_SHARED;
 	}
 
-	xfs_ilock(ip, iolock);
-
+	if (!xfs_ilock_nowait(ip, iolock)) {
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			return -EAGAIN;
+		xfs_ilock(ip, iolock);
+	}
 	ret = xfs_file_aio_write_checks(iocb, from, &iolock);
 	if (ret)
 		goto out;
@@ -553,9 +556,15 @@ xfs_file_dio_aio_write(
 	 * otherwise demote the lock if we had to take the exclusive lock
 	 * for other reasons in xfs_file_aio_write_checks.
 	 */
-	if (unaligned_io)
-		inode_dio_wait(inode);
-	else if (iolock == XFS_IOLOCK_EXCL) {
+	if (unaligned_io) {
+		/* If we are going to wait for other DIO to finish, bail */
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			if (atomic_read(&inode->i_dio_count))
+				return -EAGAIN;
+		} else {
+			inode_dio_wait(inode);
+		}
+	} else if (iolock == XFS_IOLOCK_EXCL) {
 		xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
 		iolock = XFS_IOLOCK_SHARED;
 	}
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 288ee5b840d7..9baa65eeae9e 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -1015,6 +1015,15 @@ xfs_file_iomap_begin(
 
 	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
 		if (flags & IOMAP_DIRECT) {
+			/*
+			 * A reflinked inode will result in CoW alloc.
+			 * FIXME: It could still overwrite on unshared extents
+			 * and not need allocation.
+			 */
+			if (flags & IOMAP_NOWAIT) {
+				error = -EAGAIN;
+				goto out_unlock;
+			}
 			/* may drop and re-acquire the ilock */
 			error = xfs_reflink_allocate_cow(ip, &imap, &shared,
 					&lockmode);
@@ -1032,6 +1041,14 @@ xfs_file_iomap_begin(
 
 	if ((flags & IOMAP_WRITE) && imap_needs_alloc(inode, &imap, nimaps)) {
 		/*
+		 * If nowait is set bail since we are going to make
+		 * allocations.
+		 */
+		if (flags & IOMAP_NOWAIT) {
+			error = -EAGAIN;
+			goto out_unlock;
+		}
+		/*
 		 * We cap the maximum length we map here to MAX_WRITEBACK_PAGES
 		 * pages to keep the chunks of work done where somewhat symmetric
 		 * with the work writeback does. This is a completely arbitrary
-- 
2.12.0

[PATCH 8/8] nowait aio: btrfs

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Return EAGAIN if any of the following checks fail
 + i_rwsem is not lockable
 + NODATACOW or PREALLOC is not set
 + Cannot nocow at the desired location
 + Writing beyond end of file which is not allocated

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Acked-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/file.c  | 25 ++++++++++++++++++++-----
 fs/btrfs/inode.c |  3 +++
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 520cb7230b2d..a870e5dd2b4d 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1823,12 +1823,29 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	ssize_t num_written = 0;
 	bool sync = (file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host);
 	ssize_t err;
-	loff_t pos;
-	size_t count;
+	loff_t pos = iocb->ki_pos;
+	size_t count = iov_iter_count(from);
 	loff_t oldsize;
 	int clean_page = 0;
 
-	inode_lock(inode);
+	if ((iocb->ki_flags & IOCB_NOWAIT) &&
+			(iocb->ki_flags & IOCB_DIRECT)) {
+		/* Don't sleep on inode rwsem */
+		if (!inode_trylock(inode))
+			return -EAGAIN;
+		/*
+		 * We will allocate space in case nodatacow is not set,
+		 * so bail
+		 */
+		if (!(BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
+					      BTRFS_INODE_PREALLOC)) ||
+		    check_can_nocow(BTRFS_I(inode), pos, &count) <= 0) {
+			inode_unlock(inode);
+			return -EAGAIN;
+		}
+	} else
+		inode_lock(inode);
+
 	err = generic_write_checks(iocb, from);
 	if (err <= 0) {
 		inode_unlock(inode);
@@ -1862,8 +1879,6 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	 */
 	update_time_for_write(inode);
 
-	pos = iocb->ki_pos;
-	count = iov_iter_count(from);
 	start_pos = round_down(pos, fs_info->sectorsize);
 	oldsize = i_size_read(inode);
 	if (start_pos > oldsize) {
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 5e71f1ea3391..47d3fcd86979 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -8625,6 +8625,9 @@ static ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 			dio_data.overwrite = 1;
 			inode_unlock(inode);
 			relock = true;
+		} else if (iocb->ki_flags & IOCB_NOWAIT) {
+			ret = -EAGAIN;
+			goto out;
 		}
 		ret = btrfs_delalloc_reserve_space(inode, offset, count);
 		if (ret)
-- 
2.12.0

Re: [PATCH 6/8] nowait aio: ext4

[Jan Kara]

On Tue 09-05-17 07:22:17, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> Return EAGAIN if any of the following checks fail for direct I/O:
>   + i_rwsem is lockable
>   + Writing beyond end of file (will trigger allocation)
>   + Blocks are not allocated at the write location
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

The patch looks good. You can add:

Reviewed-by: Jan Kara <jack@suse.cz>

								Honza

> ---
>  fs/ext4/file.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index cefa9835f275..2efdc6d4d3e8 100644
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -216,7 +216,13 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  		return ext4_dax_write_iter(iocb, from);
>  #endif
>  
> -	inode_lock(inode);
> +	if (iocb->ki_flags & IOCB_NOWAIT) {
> +		if (!inode_trylock(inode))
> +			return -EAGAIN;
> +	} else {
> +		inode_lock(inode);
> +	}
> +
>  	ret = ext4_write_checks(iocb, from);
>  	if (ret <= 0)
>  		goto out;
> @@ -235,9 +241,15 @@ ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  
>  	iocb->private = &overwrite;
>  	/* Check whether we do a DIO overwrite or not */
> -	if (o_direct && ext4_should_dioread_nolock(inode) && !unaligned_aio &&
> -	    ext4_overwrite_io(inode, iocb->ki_pos, iov_iter_count(from)))
> -		overwrite = 1;
> +	if (o_direct && !unaligned_aio) {
> +		if (ext4_overwrite_io(inode, iocb->ki_pos, iov_iter_count(from))) {
> +			if (ext4_should_dioread_nolock(inode))
> +				overwrite = 1;
> +		} else if (iocb->ki_flags & IOCB_NOWAIT) {
> +			ret = -EAGAIN;
> +			goto out;
> +		}
> +	}
>  
>  	ret = __generic_file_write_iter(iocb, from);
>  	inode_unlock(inode);
> -- 
> 2.12.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 1/8] Use RWF_* flags for AIO operations

[Christoph Hellwig]

Please add subsystem prefixes to your subject lines, e.g.

fs:

for all the generic fs ones,

xfs:

for XFS,

block:

for block layer changes, etc.

>  
> -	if (flags & ~(RWF_HIPRI | RWF_DSYNC | RWF_SYNC))
> -		return -EOPNOTSUPP;
> -
>  	init_sync_kiocb(&kiocb, filp);
> -	if (flags & RWF_HIPRI)
> -		kiocb.ki_flags |= IOCB_HIPRI;
> -	if (flags & RWF_DSYNC)
> -		kiocb.ki_flags |= IOCB_DSYNC;
> -	if (flags & RWF_SYNC)
> -		kiocb.ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
> +	ret = kiocb_set_rw_flags(&kiocb, flags);
> +	if (ret)
> +		return ret;

And please split factoring out kiocb_set_rw_flags into a separate
prep patch.

Otherwise these changes look fine:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 2/8] nowait aio: Introduce RWF_NOWAIT

[Christoph Hellwig]

On Tue, May 09, 2017 at 07:22:13AM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> This flag informs kernel to bail out if an AIO request will block
> for reasons such as file allocations, or a writeback triggered,
> or would block while allocating requests while performing
> direct I/O.
> 
> Unfortunately, aio_flags is not checked for validity, which would
> break existing applications which have it set to anything besides zero
> or IOCB_FLAG_RESFD. So, we are using aio_reserved1 and renaming it
> to aio_rw_flags.
> 
> RWF_NOWAIT is translated to IOCB_NOWAIT for iocb->ki_flags.
> 
> The check for -EOPNOTSUPP is placed in generic_file_write_iter(). This
> is called by most filesystems, either through fsops.write_iter() or through
> the function defined by write_iter(). If not, we perform the check defined
> by .write_iter() which is called for direct IO specifically.
> 
> Filesystems xfs, btrfs and ext4 would be supported in the following patches.
> 
> Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
> ---
>  fs/9p/vfs_file.c        | 3 +++
>  fs/aio.c                | 6 ++++++
>  fs/ceph/file.c          | 3 +++
>  fs/cifs/file.c          | 3 +++
>  fs/fuse/file.c          | 3 +++
>  fs/nfs/direct.c         | 3 +++
>  fs/ocfs2/file.c         | 3 +++
>  include/linux/fs.h      | 5 ++++-
>  include/uapi/linux/fs.h | 1 +
>  mm/filemap.c            | 3 +++
>  10 files changed, 32 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/9p/vfs_file.c b/fs/9p/vfs_file.c
> index 3de3b4a89d89..403681db7723 100644
> --- a/fs/9p/vfs_file.c
> +++ b/fs/9p/vfs_file.c
> @@ -411,6 +411,9 @@ v9fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	loff_t origin;
>  	int err = 0;
>  
> +	if (iocb->ki_flags & IOCB_NOWAIT)
> +		return -EOPNOTSUPP;
> +
>  	retval = generic_write_checks(iocb, from);
>  	if (retval <= 0)
>  		return retval;
> diff --git a/fs/aio.c b/fs/aio.c
> index 020fa0045e3c..ea9f8581d902 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1592,6 +1592,12 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
>  		goto out_put_req;
>  	}
>  
> +	if ((req->common.ki_flags & IOCB_NOWAIT) &&
> +		!(req->common.ki_flags & IOCB_DIRECT)) {

Weird indentation.  Either align after the opening if brace:

	if ((req->common.ki_flags & IOCB_NOWAIT) &&
	    !(req->common.ki_flags & IOCB_DIRECT)) {

or using two tabs:

	if ((req->common.ki_flags & IOCB_NOWAIT) &&
			!(req->common.ki_flags & IOCB_DIRECT)) {

if the first version looks confusing, but never using the same
indentation level as the following code.

Except for that the patch looks fine to me:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Christoph Hellwig]

It might make sense to move filemap_range_has_page into a separate
prep patch.

Otherwise this looks fine:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 5/8] nowait aio: return on congested block device

[Christoph Hellwig]

Looks fine,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Although lifting the make_request limit is something a lot of users
would appreciate in the near future..

Re: [PATCH 7/8] nowait aio: xfs

[Christoph Hellwig]

Looks fine,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH 5/8] nowait aio: return on congested block device

[Goldwyn Rodrigues]

On 05/11/2017 02:44 AM, Christoph Hellwig wrote:
> Looks fine,
> 
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> 
> Although lifting the make_request limit is something a lot of users
> would appreciate in the near future..
> 

Yes, I understand. That will be on my todo list next on priority.

-- 
Goldwyn

[PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Find out if the write will trigger a wait due to writeback. If yes,
return -EAGAIN.

This introduces a new function filemap_range_has_page() which
returns true if the file's mapping has a page within the range
mentioned.

Return -EINVAL for buffered AIO: there are multiple causes of
delay such as page locks, dirty throttling logic, page loading
from disk etc. which cannot be taken care of.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 include/linux/fs.h |  2 ++
 mm/filemap.c       | 50 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e44de1c981a0..b14eab5daeb2 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2514,6 +2514,8 @@ extern int filemap_fdatawait(struct address_space *);
 extern void filemap_fdatawait_keep_errors(struct address_space *);
 extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
 				   loff_t lend);
+extern int filemap_range_has_page(struct address_space *, loff_t lstart,
+				   loff_t lend);
 extern int filemap_write_and_wait(struct address_space *mapping);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);
diff --git a/mm/filemap.c b/mm/filemap.c
index d51670b7fe6b..48b83d1d4a30 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -376,6 +376,39 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);
 
+/**
+ * filemap_range_has_page - check if a page exists in range.
+ * @mapping:		address space structure to wait for
+ * @start_byte:		offset in bytes where the range starts
+ * @end_byte:		offset in bytes where the range ends (inclusive)
+ *
+ * Find at least one page in the range supplied, usually used to check if
+ * direct writing in this range will trigger a writeback.
+ */
+int filemap_range_has_page(struct address_space *mapping,
+				loff_t start_byte, loff_t end_byte)
+{
+	pgoff_t index = start_byte >> PAGE_SHIFT;
+	pgoff_t end = end_byte >> PAGE_SHIFT;
+	struct pagevec pvec;
+	int ret;
+
+	if (end_byte < start_byte)
+		return 0;
+
+	if (mapping->nrpages == 0)
+		return 0;
+
+	pagevec_init(&pvec, 0);
+	ret = pagevec_lookup(&pvec, mapping, index, 1);
+	if (!ret)
+		return 0;
+	ret = (pvec.pages[0]->index <= end);
+	pagevec_release(&pvec);
+	return ret;
+}
+EXPORT_SYMBOL(filemap_range_has_page);
+
 static int __filemap_fdatawait_range(struct address_space *mapping,
 				     loff_t start_byte, loff_t end_byte)
 {
@@ -2640,6 +2673,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 
 	pos = iocb->ki_pos;
 
+	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
+		return -EINVAL;
+
 	if (limit != RLIM_INFINITY) {
 		if (iocb->ki_pos >= limit) {
 			send_sig(SIGXFSZ, current, 0);
@@ -2709,9 +2745,17 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	write_len = iov_iter_count(from);
 	end = (pos + write_len - 1) >> PAGE_SHIFT;
 
-	written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
-	if (written)
-		goto out;
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		/* If there are pages to writeback, return */
+		if (filemap_range_has_page(inode->i_mapping, pos,
+					   pos + iov_iter_count(from)))
+			return -EAGAIN;
+	} else {
+		written = filemap_write_and_wait_range(mapping, pos,
+							pos + write_len - 1);
+		if (written)
+			goto out;
+	}
 
 	/*
 	 * After a write we want buffered reads to be sure to go to disk to get
-- 
2.12.0

[PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Find out if the write will trigger a wait due to writeback. If yes,
return -EAGAIN.

This introduces a new function filemap_range_has_page() which
returns true if the file's mapping has a page within the range
mentioned.

Return -EINVAL for buffered AIO: there are multiple causes of
delay such as page locks, dirty throttling logic, page loading
from disk etc. which cannot be taken care of.
---
 include/linux/fs.h |  2 ++
 mm/filemap.c       | 50 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 802cfe2..4721136 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2515,6 +2515,8 @@ extern int filemap_fdatawait(struct address_space *);
 extern void filemap_fdatawait_keep_errors(struct address_space *);
 extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
 				   loff_t lend);
+extern int filemap_range_has_page(struct address_space *, loff_t lstart,
+				   loff_t lend);
 extern int filemap_write_and_wait(struct address_space *mapping);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);
diff --git a/mm/filemap.c b/mm/filemap.c
index e08f3b9..c020e23 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -376,6 +376,39 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);
 
+/**
+ * filemap_range_has_page - check if a page exists in range.
+ * @mapping:		address space structure to wait for
+ * @start_byte:		offset in bytes where the range starts
+ * @end_byte:		offset in bytes where the range ends (inclusive)
+ *
+ * Find at least one page in the range supplied, usually used to check if
+ * direct writing in this range will trigger a writeback.
+ */
+int filemap_range_has_page(struct address_space *mapping,
+				loff_t start_byte, loff_t end_byte)
+{
+	pgoff_t index = start_byte >> PAGE_SHIFT;
+	pgoff_t end = end_byte >> PAGE_SHIFT;
+	struct pagevec pvec;
+	int ret;
+
+	if (end_byte < start_byte)
+		return 0;
+
+	if (mapping->nrpages == 0)
+		return 0;
+
+	pagevec_init(&pvec, 0);
+	ret = pagevec_lookup(&pvec, mapping, index, 1);
+	if (!ret)
+		return 0;
+	ret = (pvec.pages[0]->index <= end);
+	pagevec_release(&pvec);
+	return ret;
+}
+EXPORT_SYMBOL(filemap_range_has_page);
+
 static int __filemap_fdatawait_range(struct address_space *mapping,
 				     loff_t start_byte, loff_t end_byte)
 {
@@ -2640,6 +2673,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 
 	pos = iocb->ki_pos;
 
+	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
+		return -EINVAL;
+
 	if (limit != RLIM_INFINITY) {
 		if (iocb->ki_pos >= limit) {
 			send_sig(SIGXFSZ, current, 0);
@@ -2709,9 +2745,17 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	write_len = iov_iter_count(from);
 	end = (pos + write_len - 1) >> PAGE_SHIFT;
 
-	written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
-	if (written)
-		goto out;
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		/* If there are pages to writeback, return */
+		if (filemap_range_has_page(inode->i_mapping, pos,
+					   pos + iov_iter_count(from)))
+			return -EAGAIN;
+	} else {
+		written = filemap_write_and_wait_range(mapping, pos,
+							pos + write_len - 1);
+		if (written)
+			goto out;
+	}
 
 	/*
 	 * After a write we want buffered reads to be sure to go to disk to get
-- 
2.10.2

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

On 03/16/2017 08:20 AM, Matthew Wilcox wrote:
> On Wed, Mar 15, 2017 at 04:51:02PM -0500, Goldwyn Rodrigues wrote:
>> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
>>
>> Find out if the write will trigger a wait due to writeback. If yes,
>> return -EAGAIN.
>>
>> This introduces a new function filemap_range_has_page() which
>> returns true if the file's mapping has a page within the range
>> mentioned.
>>
>> Return -EINVAL for buffered AIO: there are multiple causes of
>> delay such as page locks, dirty throttling logic, page loading
>> from disk etc. which cannot be taken care of.
> 
> Also, this patch only touches the write path; we have a similar call to
> write_and_wait_range() in generic_file_read_iter().
> 

This patch series is concerned with direct-write AIO paths only.

-- 
Goldwyn

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

On 03/16/2017 08:08 AM, Matthew Wilcox wrote:
> On Wed, Mar 15, 2017 at 04:51:02PM -0500, Goldwyn Rodrigues wrote:
>> This introduces a new function filemap_range_has_page() which
>> returns true if the file's mapping has a page within the range
>> mentioned.
> 
> I thought you were going to replace this patch with one that starts
> writeback for these pages but does not wait for them?
> 

As mentioned by Jan, Flags to filemap_write_and_wait_range are
unnecessarily complicated. The AIO-DIO API users who eye for performance
usually are careful with page writes/evictions. As a fallback, they can
(and should) go the wait route (without  IOCB_RW_FLAG_NOWAIT).

Finally, my take on this is that we don't want to perform tasks for a
following system call, which may or may not immediately follow the
current one. May not, because an application (DB) will offload the task
from the CPU thread to the I/O thread in case of -EAGAIN. A system call
should be complete in itself (and do the minimum, what is asked).


-- 
Goldwyn

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Matthew Wilcox]

On Wed, Mar 15, 2017 at 04:51:02PM -0500, Goldwyn Rodrigues wrote:
> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
> 
> Find out if the write will trigger a wait due to writeback. If yes,
> return -EAGAIN.
> 
> This introduces a new function filemap_range_has_page() which
> returns true if the file's mapping has a page within the range
> mentioned.
> 
> Return -EINVAL for buffered AIO: there are multiple causes of
> delay such as page locks, dirty throttling logic, page loading
> from disk etc. which cannot be taken care of.

Also, this patch only touches the write path; we have a similar call to
write_and_wait_range() in generic_file_read_iter().

Actually, why do we even have that?  Why can't we satisfy an O_DIRECT
read from the cache?

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Matthew Wilcox]

On Wed, Mar 15, 2017 at 04:51:02PM -0500, Goldwyn Rodrigues wrote:
> This introduces a new function filemap_range_has_page() which
> returns true if the file's mapping has a page within the range
> mentioned.

I thought you were going to replace this patch with one that starts
writeback for these pages but does not wait for them?

[PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Find out if the write will trigger a wait due to writeback. If yes,
return -EAGAIN.

This introduces a new function filemap_range_has_page() which
returns true if the file's mapping has a page within the range
mentioned.

Return -EINVAL for buffered AIO: there are multiple causes of
delay such as page locks, dirty throttling logic, page loading
from disk etc. which cannot be taken care of.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 include/linux/fs.h |  2 ++
 mm/filemap.c       | 50 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index e8d9346..4a30e8f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2514,6 +2514,8 @@ extern int filemap_fdatawait(struct address_space *);
 extern void filemap_fdatawait_keep_errors(struct address_space *);
 extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
 				   loff_t lend);
+extern int filemap_range_has_page(struct address_space *, loff_t lstart,
+				   loff_t lend);
 extern int filemap_write_and_wait(struct address_space *mapping);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);
diff --git a/mm/filemap.c b/mm/filemap.c
index e08f3b9..c020e23 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -376,6 +376,39 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);
 
+/**
+ * filemap_range_has_page - check if a page exists in range.
+ * @mapping:		address space structure to wait for
+ * @start_byte:		offset in bytes where the range starts
+ * @end_byte:		offset in bytes where the range ends (inclusive)
+ *
+ * Find at least one page in the range supplied, usually used to check if
+ * direct writing in this range will trigger a writeback.
+ */
+int filemap_range_has_page(struct address_space *mapping,
+				loff_t start_byte, loff_t end_byte)
+{
+	pgoff_t index = start_byte >> PAGE_SHIFT;
+	pgoff_t end = end_byte >> PAGE_SHIFT;
+	struct pagevec pvec;
+	int ret;
+
+	if (end_byte < start_byte)
+		return 0;
+
+	if (mapping->nrpages == 0)
+		return 0;
+
+	pagevec_init(&pvec, 0);
+	ret = pagevec_lookup(&pvec, mapping, index, 1);
+	if (!ret)
+		return 0;
+	ret = (pvec.pages[0]->index <= end);
+	pagevec_release(&pvec);
+	return ret;
+}
+EXPORT_SYMBOL(filemap_range_has_page);
+
 static int __filemap_fdatawait_range(struct address_space *mapping,
 				     loff_t start_byte, loff_t end_byte)
 {
@@ -2640,6 +2673,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 
 	pos = iocb->ki_pos;
 
+	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
+		return -EINVAL;
+
 	if (limit != RLIM_INFINITY) {
 		if (iocb->ki_pos >= limit) {
 			send_sig(SIGXFSZ, current, 0);
@@ -2709,9 +2745,17 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	write_len = iov_iter_count(from);
 	end = (pos + write_len - 1) >> PAGE_SHIFT;
 
-	written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
-	if (written)
-		goto out;
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		/* If there are pages to writeback, return */
+		if (filemap_range_has_page(inode->i_mapping, pos,
+					   pos + iov_iter_count(from)))
+			return -EAGAIN;
+	} else {
+		written = filemap_write_and_wait_range(mapping, pos,
+							pos + write_len - 1);
+		if (written)
+			goto out;
+	}
 
 	/*
 	 * After a write we want buffered reads to be sure to go to disk to get
-- 
2.10.2

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Jan Kara]

On Thu 02-03-17 06:12:45, Matthew Wilcox wrote:
> On Thu, Mar 02, 2017 at 11:38:45AM +0100, Jan Kara wrote:
> > On Wed 01-03-17 07:38:57, Christoph Hellwig wrote:
> > > On Tue, Feb 28, 2017 at 07:46:06PM -0800, Matthew Wilcox wrote:
> > > > But what's going to kick these pages out of cache?  Shouldn't we rather
> > > > find the pages, kick them out if clean, start writeback if not, and *then*
> > > > return -EAGAIN?
> > > 
> > > As pointed out in the last round of these patches I think we really
> > > need to pass a flags argument to filemap_write_and_wait_range to
> > > communicate the non-blocking nature and only return -EAGAIN if we'd
> > > block.  As a bonus that can indeed start to kick the pages out.
> > 
> > Aren't flags to filemap_write_and_wait_range() unnecessary complication?
> > Realistically, most users wanting performance from AIO DIO so badly that
> > they bother with this API won't have any pages to write / evict. If they do
> > by some bad accident, they can fall back to standard "blocking" AIO DIO.
> > So I don't see much value in teaching filemap_write_and_wait_range() about
> > a non-blocking mode...
> 
> That lets me execute a DoS against a user using this API.  All I have
> to do is open the file they're using read-only and read a byte from it.
> Page goes into page-cache, and they'll only get -EAGAIN from calling
> this syscall until the page ages out.

It will not be a DoS. This non-blocking AIO can always return EAGAIN when
it feels like it and the caller is required to fall back to a blocking
version in that case if he wants to guarantee forward progress. It is just
a performance optimization which allows user (database) to submit IO from a
computation thread instead of having to offload it to an IO thread...

> Also, I don't understand why this is a flag.  Isn't the point of AIO to
> be non-blocking?  Why isn't this just a change to how we do AIO?

Because this is an API change and the caller has to implement some handling
to guarantee a forward progress of non-blocking IO...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Matthew Wilcox]

On Thu, Mar 02, 2017 at 11:38:45AM +0100, Jan Kara wrote:
> On Wed 01-03-17 07:38:57, Christoph Hellwig wrote:
> > On Tue, Feb 28, 2017 at 07:46:06PM -0800, Matthew Wilcox wrote:
> > > But what's going to kick these pages out of cache?  Shouldn't we rather
> > > find the pages, kick them out if clean, start writeback if not, and *then*
> > > return -EAGAIN?
> > 
> > As pointed out in the last round of these patches I think we really
> > need to pass a flags argument to filemap_write_and_wait_range to
> > communicate the non-blocking nature and only return -EAGAIN if we'd
> > block.  As a bonus that can indeed start to kick the pages out.
> 
> Aren't flags to filemap_write_and_wait_range() unnecessary complication?
> Realistically, most users wanting performance from AIO DIO so badly that
> they bother with this API won't have any pages to write / evict. If they do
> by some bad accident, they can fall back to standard "blocking" AIO DIO.
> So I don't see much value in teaching filemap_write_and_wait_range() about
> a non-blocking mode...

That lets me execute a DoS against a user using this API.  All I have
to do is open the file they're using read-only and read a byte from it.
Page goes into page-cache, and they'll only get -EAGAIN from calling
this syscall until the page ages out.

Also, I don't understand why this is a flag.  Isn't the point of AIO to
be non-blocking?  Why isn't this just a change to how we do AIO?

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Jan Kara]

On Wed 01-03-17 07:38:57, Christoph Hellwig wrote:
> On Tue, Feb 28, 2017 at 07:46:06PM -0800, Matthew Wilcox wrote:
> > Ugh, this is pretty inefficient.  If that's all you want to know, then
> > using the radix tree directly will be far more efficient than spinning
> > up all the pagevec machinery only to discard the pages found.
> > 
> > But what's going to kick these pages out of cache?  Shouldn't we rather
> > find the pages, kick them out if clean, start writeback if not, and *then*
> > return -EAGAIN?
> > 
> > So maybe we want to spin up the pagevec machinery after all so we can
> > do that extra work?
> 
> As pointed out in the last round of these patches I think we really
> need to pass a flags argument to filemap_write_and_wait_range to
> communicate the non-blocking nature and only return -EAGAIN if we'd
> block.  As a bonus that can indeed start to kick the pages out.

Aren't flags to filemap_write_and_wait_range() unnecessary complication?
Realistically, most users wanting performance from AIO DIO so badly that
they bother with this API won't have any pages to write / evict. If they do
by some bad accident, they can fall back to standard "blocking" AIO DIO.
So I don't see much value in teaching filemap_write_and_wait_range() about
a non-blocking mode...

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Christoph Hellwig]

On Tue, Feb 28, 2017 at 07:46:06PM -0800, Matthew Wilcox wrote:
> Ugh, this is pretty inefficient.  If that's all you want to know, then
> using the radix tree directly will be far more efficient than spinning
> up all the pagevec machinery only to discard the pages found.
> 
> But what's going to kick these pages out of cache?  Shouldn't we rather
> find the pages, kick them out if clean, start writeback if not, and *then*
> return -EAGAIN?
> 
> So maybe we want to spin up the pagevec machinery after all so we can
> do that extra work?

As pointed out in the last round of these patches I think we really
need to pass a flags argument to filemap_write_and_wait_range to
communicate the non-blocking nature and only return -EAGAIN if we'd
block.  As a bonus that can indeed start to kick the pages out.

Re: [PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Matthew Wilcox]

On Tue, Feb 28, 2017 at 05:36:05PM -0600, Goldwyn Rodrigues wrote:
> Find out if the write will trigger a wait due to writeback. If yes,
> return -EAGAIN.
> 
> This introduces a new function filemap_range_has_page() which
> returns true if the file's mapping has a page within the range
> mentioned.

Ugh, this is pretty inefficient.  If that's all you want to know, then
using the radix tree directly will be far more efficient than spinning
up all the pagevec machinery only to discard the pages found.

But what's going to kick these pages out of cache?  Shouldn't we rather
find the pages, kick them out if clean, start writeback if not, and *then*
return -EAGAIN?

So maybe we want to spin up the pagevec machinery after all so we can
do that extra work?

[PATCH 3/8] nowait aio: return if direct write will trigger writeback

[Goldwyn Rodrigues]

From: Goldwyn Rodrigues <rgoldwyn@suse.com>

Find out if the write will trigger a wait due to writeback. If yes,
return -EAGAIN.

This introduces a new function filemap_range_has_page() which
returns true if the file's mapping has a page within the range
mentioned.

Return -EINVAL for buffered AIO: there are multiple causes of
delay such as page locks, dirty throttling logic, page loading
from disk etc. which cannot be taken care of.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
---
 include/linux/fs.h |  2 ++
 mm/filemap.c       | 50 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index ab2f556..527ef53 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2494,6 +2494,8 @@ extern int filemap_fdatawait(struct address_space *);
 extern void filemap_fdatawait_keep_errors(struct address_space *);
 extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
 				   loff_t lend);
+extern int filemap_range_has_page(struct address_space *, loff_t lstart,
+				   loff_t lend);
 extern int filemap_write_and_wait(struct address_space *mapping);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);
diff --git a/mm/filemap.c b/mm/filemap.c
index 78dd50e..82335f4 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -375,6 +375,39 @@ int filemap_flush(struct address_space *mapping)
 }
 EXPORT_SYMBOL(filemap_flush);
 
+/**
+ * filemap_range_has_page - check if a page exists in range.
+ * @mapping:		address space structure to wait for
+ * @start_byte:		offset in bytes where the range starts
+ * @end_byte:		offset in bytes where the range ends (inclusive)
+ *
+ * Find at least one page in the range supplied, usually used to check if
+ * direct writing in this range will trigger a writeback.
+ */
+int filemap_range_has_page(struct address_space *mapping,
+				loff_t start_byte, loff_t end_byte)
+{
+	pgoff_t index = start_byte >> PAGE_SHIFT;
+	pgoff_t end = end_byte >> PAGE_SHIFT;
+	struct pagevec pvec;
+	int ret;
+
+	if (end_byte < start_byte)
+		return 0;
+
+	if (mapping->nrpages == 0)
+		return 0;
+
+	pagevec_init(&pvec, 0);
+	ret = pagevec_lookup(&pvec, mapping, index, 1);
+	if (!ret)
+		return 0;
+	ret = (pvec.pages[0]->index <= end);
+	pagevec_release(&pvec);
+	return ret;
+}
+EXPORT_SYMBOL(filemap_range_has_page);
+
 static int __filemap_fdatawait_range(struct address_space *mapping,
 				     loff_t start_byte, loff_t end_byte)
 {
@@ -2631,6 +2664,9 @@ inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 
 	pos = iocb->ki_pos;
 
+	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
+		return -EINVAL;
+
 	if (limit != RLIM_INFINITY) {
 		if (iocb->ki_pos >= limit) {
 			send_sig(SIGXFSZ, current, 0);
@@ -2700,9 +2736,17 @@ generic_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 	write_len = iov_iter_count(from);
 	end = (pos + write_len - 1) >> PAGE_SHIFT;
 
-	written = filemap_write_and_wait_range(mapping, pos, pos + write_len - 1);
-	if (written)
-		goto out;
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		/* If there are pages to writeback, return */
+		if (filemap_range_has_page(inode->i_mapping, pos,
+					   pos + iov_iter_count(from)))
+			return -EAGAIN;
+	} else {
+		written = filemap_write_and_wait_range(mapping, pos,
+							pos + write_len - 1);
+		if (written)
+			goto out;
+	}
 
 	/*
 	 * After a write we want buffered reads to be sure to go to disk to get
-- 
2.10.2