[PATCH] direct-to-BIO for O_DIRECT

Here's a patch which converts O_DIRECT to go direct-to-BIO, bypassing
the kiovec layer.  It's followed by a patch which converts the raw
driver to use the O_DIRECT engine.

CPU utilisation is about the same as the kiovec-based implementation.
Read and write bandwidth are the same too, for 128k chunks.   But with
one megabyte chunks, this implementation is 20% faster at writing.

I assume this is because the kiobuf-based implementation has to stop
and wait for each 128k chunk, whereas this code streams the entire
request, regardless of its size.

This is with a single (oldish) scsi disk on aic7xxx.  I'd expect the
margin to widen on higher-end hardware which likes to have more
requests in flight.

Question is: what do we want to do with this sucker?  These are the
remaining users of kiovecs:

	drivers/md/lvm-snap.c
	drivers/media/video/video-buf.c
	drivers/mtd/devices/blkmtd.c
	drivers/scsi/sg.c

the video and mtd drivers seems to be fairly easy to de-kiobufize.
I'm aware of one proprietary driver which uses kiobufs.  XFS uses
kiobufs a little bit - just to map the pages.

So with a bit of effort and maintainer-irritation, we can extract
the kiobuf layer from the kernel.

5 files changed, 502 insertions, 7 deletions

diff --git a/fs/Makefile b/fs/Makefile
index 2f516179753c..f96955eac7b9 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -15,7 +15,7 @@ obj-y :=	open.o read_write.o devices.o file_table.o buffer.o \
 		namei.o fcntl.o ioctl.o readdir.o select.o fifo.o locks.o \
 		dcache.o inode.o attr.o bad_inode.o file.o iobuf.o dnotify.o \
 		filesystems.o namespace.o seq_file.o xattr.o libfs.o \
-		fs-writeback.o mpage.o
+		fs-writeback.o mpage.o direct-io.o
 
 ifneq ($(CONFIG_NFSD),n)
 ifneq ($(CONFIG_NFSD),)
diff --git a/fs/block_dev.c b/fs/block_dev.c
index ec7e816c7c0f..50ee35de5e11 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -106,9 +106,12 @@ static int blkdev_get_block(struct inode * inode, sector_t iblock, struct buffer
 	return 0;
 }
 
-static int blkdev_direct_IO(int rw, struct inode * inode, struct kiobuf * iobuf, unsigned long blocknr, int blocksize)
+static int
+blkdev_direct_IO(int rw, struct inode *inode, char *buf,
+			loff_t offset, size_t count)
 {
-	return generic_direct_IO(rw, inode, iobuf, blocknr, blocksize, blkdev_get_block);
+	return generic_direct_IO(rw, inode, buf, offset,
+				count, blkdev_get_block);
 }
 
 static int blkdev_writepage(struct page * page)
diff --git a/fs/buffer.c b/fs/buffer.c
index f96f5f48bd13..07ab2f0108bb 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2311,6 +2311,7 @@ sector_t generic_block_bmap(struct address_space *mapping, sector_t block,
 	return tmp.b_blocknr;
 }
 
+#if 0
 int generic_direct_IO(int rw, struct inode *inode,
 			struct kiobuf *iobuf, unsigned long blocknr,
 			int blocksize, get_block_t *get_block)
@@ -2355,6 +2356,7 @@ int generic_direct_IO(int rw, struct inode *inode,
  out:
 	return retval;
 }
+#endif
 
 /*
  * Start I/O on a physical range of kernel memory, defined by a vector
diff --git a/fs/direct-io.c b/fs/direct-io.c
new file mode 100644
index 000000000000..32c2b1097540
--- /dev/null
+++ b/fs/direct-io.c
@@ -0,0 +1,491 @@
+/*
+ * mm/direct-io.c
+ *
+ * Copyright (C) 2002, Linus Torvalds.
+ *
+ * O_DIRECT
+ *
+ * 04Jul2002	akpm@zip.com.au
+ *		Initial version
+ */
+
+#include <linux/kernel.h>
+#include <linux/types.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/bio.h>
+#include <linux/wait.h>
+#include <linux/err.h>
+#include <linux/buffer_head.h>
+#include <linux/rwsem.h>
+#include <asm/atomic.h>
+
+/*
+ * The largest-sized BIO which this code will assemble, in bytes.  Set this
+ * to PAGE_SIZE if your drivers are broken.
+ */
+#define DIO_BIO_MAX_SIZE BIO_MAX_SIZE
+
+/*
+ * How many user pages to map in one call to get_user_pages().  This determines
+ * the size of a structure on the stack.
+ */
+#define DIO_PAGES	64
+
+struct dio {
+	/* BIO submission state */
+	struct bio *bio;		/* bio under assembly */
+	struct bio_vec *bvec;		/* current bvec in that bio */
+	struct inode *inode;
+	int rw;
+	sector_t block_in_file;		/* changes */
+	sector_t final_block_in_request;/* doesn't change */
+	unsigned first_block_in_page;	/* doesn't change */
+	int boundary;			/* prev block is at a boundary */
+	int reap_counter;		/* rate limit reaping */
+	get_block_t *get_block;
+	sector_t last_block_in_bio;
+
+	/* Page fetching state */
+	int curr_page;			/* changes */
+	int total_pages;		/* doesn't change */
+	unsigned long curr_user_address;/* changes */
+
+	/* Page queue */
+	struct page *pages[DIO_PAGES];
+	unsigned head;
+	unsigned tail;
+
+	/* BIO completion state */
+	atomic_t bio_count;
+	spinlock_t bio_list_lock;
+	struct bio *bio_list;		/* singly linked via bi_private */
+	wait_queue_head_t wait_q;
+};
+
+/*
+ * How many pages are in the queue?
+ */
+static inline unsigned dio_pages_present(struct dio *dio)
+{
+	return dio->head - dio->tail;
+}
+
+/*
+ * Go grab and pin some userspace pages.   Typically we'll get 64 at a time.
+ */
+static int dio_refill_pages(struct dio *dio)
+{
+	int ret;
+	int nr_pages;
+
+	nr_pages = min(dio->total_pages - dio->curr_page, DIO_PAGES);
+	ret = get_user_pages(
+		current,			/* Task for fault acounting */
+		current->mm,			/* whose pages? */
+		dio->curr_user_address,		/* Where from? */
+		nr_pages,			/* How many pages? */
+		dio->rw == READ,		/* Write to memory? */
+		0,				/* force (?) */
+		&dio->pages[0],
+		NULL);				/* vmas */
+
+	if (ret >= 0) {
+		dio->curr_user_address += ret * PAGE_SIZE;
+		dio->curr_page += ret;
+		dio->head = 0;
+		dio->tail = ret;
+		ret = 0;
+	}
+	return ret;	
+}
+
+/*
+ * Get another userspace page.  Returns an ERR_PTR on error.  Pages are
+ * buffered inside the dio so that we can call get_user_pages() against a
+ * decent number of pages, less frequently.  To provide nicer use of the
+ * L1 cache.
+ */
+static struct page *dio_get_page(struct dio *dio)
+{
+	if (dio_pages_present(dio) == 0) {
+		int ret;
+
+		ret = dio_refill_pages(dio);
+		if (ret) {
+			printk("%s: dio_refill_pages returns %d\n",
+				__FUNCTION__, ret);
+			return ERR_PTR(ret);
+		}
+		BUG_ON(dio_pages_present(dio) == 0);
+	}
+	return dio->pages[dio->head++];
+}
+
+/*
+ * The BIO completion handler simply queues the BIO up for the process-context
+ * handler.
+ *
+ * During I/O bi_private points at the dio.  After I/O, bi_private is used to
+ * implement a singly-linked list of completed BIOs, at dio->bio_list.
+ */
+static void dio_bio_end_io(struct bio *bio)
+{
+	struct dio *dio = bio->bi_private;
+	unsigned long flags;
+
+	spin_lock_irqsave(&dio->bio_list_lock, flags);
+	bio->bi_private = dio->bio_list;
+	dio->bio_list = bio;
+	spin_unlock_irqrestore(&dio->bio_list_lock, flags);
+	wake_up(&dio->wait_q);
+}
+
+static int
+dio_bio_alloc(struct dio *dio, struct block_device *bdev,
+		sector_t first_sector, int nr_vecs)
+{
+	struct bio *bio;
+
+	bio = bio_alloc(GFP_KERNEL, nr_vecs);
+	if (bio == NULL)
+		return -ENOMEM;
+
+	bio->bi_bdev = bdev;
+	bio->bi_vcnt = nr_vecs;
+	bio->bi_idx = 0;
+	bio->bi_size = 0;
+	bio->bi_sector = first_sector;
+	bio->bi_io_vec[0].bv_page = NULL;
+	bio->bi_end_io = dio_bio_end_io;
+
+	dio->bio = bio;
+	dio->bvec = NULL;		/* debug */
+	return 0;
+}
+
+static void dio_bio_submit(struct dio *dio)
+{
+	struct bio *bio = dio->bio;
+
+	bio->bi_vcnt = bio->bi_idx;
+	bio->bi_idx = 0;
+	bio->bi_private = dio;
+	atomic_inc(&dio->bio_count);
+	submit_bio(dio->rw, bio);
+
+	dio->bio = NULL;
+	dio->bvec = NULL;
+}
+
+/*
+ * Release any resources in case of a failure
+ */
+static void dio_cleanup(struct dio *dio)
+{
+	while (dio_pages_present(dio))
+		page_cache_release(dio_get_page(dio));
+}
+
+/*
+ * Wait for the next BIO to complete.  Remove it and return it.
+ */
+static struct bio *dio_await_one(struct dio *dio)
+{
+	DECLARE_WAITQUEUE(wait, current);
+	unsigned long flags;
+	struct bio *bio;
+
+	spin_lock_irqsave(&dio->bio_list_lock, flags);
+	while (dio->bio_list == NULL) {
+		add_wait_queue(&dio->wait_q, &wait);
+		set_current_state(TASK_UNINTERRUPTIBLE);
+		if (dio->bio_list == NULL) {
+			spin_unlock_irqrestore(&dio->bio_list_lock, flags);
+			blk_run_queues();
+			schedule();
+			spin_lock_irqsave(&dio->bio_list_lock, flags);
+		}
+		set_current_state(TASK_RUNNING);
+		remove_wait_queue(&dio->wait_q, &wait);
+	}
+	bio = dio->bio_list;
+	dio->bio_list = bio->bi_private;
+	spin_unlock_irqrestore(&dio->bio_list_lock, flags);
+	return bio;
+}
+
+/*
+ * Process one completed BIO.  No locks are held.
+ */
+static int dio_bio_complete(struct dio *dio, struct bio *bio)
+{
+	const int uptodate = test_bit(BIO_UPTODATE, &bio->bi_flags);
+	struct bio_vec *bvec = bio->bi_io_vec;
+	int page_no;
+	int ret = 0;
+
+	for (page_no = 0; page_no < bio->bi_vcnt; page_no++) {
+		struct page *page = bvec[page_no].bv_page;
+
+		if (!uptodate) {
+			if (ret == 0)
+				ret = -EIO;
+		}
+
+		if (dio->rw == READ)
+			set_page_dirty(page);
+		page_cache_release(page);
+	}
+	atomic_dec(&dio->bio_count);
+	bio_put(bio);
+	return ret;
+}
+
+/*
+ * Wait on and process all in-flight BIOs.
+ */
+static int dio_await_completion(struct dio *dio)
+{
+	int ret = 0;
+	while (atomic_read(&dio->bio_count)) {
+		struct bio *bio = dio_await_one(dio);
+		int ret2;
+
+		ret2 = dio_bio_complete(dio, bio);
+		if (ret == 0)
+			ret = ret2;
+	}
+	return ret;
+}
+
+/*
+ * A really large O_DIRECT read or write can generate a lot of BIOs.  So
+ * to keep the memory consumption sane we periodically reap any completed BIOs
+ * during the BIO generation phase.
+ *
+ * This also helps to limis the peak amount of pinned userspace memory.
+ */
+static int dio_bio_reap(struct dio *dio)
+{
+	int ret = 0;
+
+	if (dio->reap_counter++ >= 64) {
+		while (dio->bio_list) {
+			unsigned long flags;
+			struct bio *bio;
+			int ret2;
+
+			spin_lock_irqsave(&dio->bio_list_lock, flags);
+			bio = dio->bio_list;
+			dio->bio_list = bio->bi_private;
+			spin_unlock_irqrestore(&dio->bio_list_lock, flags);
+			ret2 = dio_bio_complete(dio, bio);
+			if (ret == 0)
+				ret = ret2;
+		}
+		dio->reap_counter = 0;
+	}
+	return ret;
+}
+
+/*
+ * Walk the user pages, and the file, mapping blocks to disk and emitting BIOs.
+ */
+int do_direct_IO(struct dio *dio)
+{
+	struct inode * const inode = dio->inode;
+	const unsigned blkbits = inode->i_blkbits;
+	const unsigned blocksize = 1 << blkbits;
+	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
+	struct page *page;
+	unsigned block_in_page;
+	int ret;
+
+	/* The I/O can start at any block offset within the first page */
+	block_in_page = dio->first_block_in_page;
+
+	while (dio->block_in_file < dio->final_block_in_request) {
+		int new_page;	/* Need to insert this page into the BIO? */
+
+		page = dio_get_page(dio);
+		if (IS_ERR(page)) {
+			ret = PTR_ERR(page);
+			goto out;
+		}
+
+		new_page = 1;
+		for ( ; block_in_page < blocks_per_page; block_in_page++) {
+			struct buffer_head map_bh;
+			struct bio *bio;
+
+			map_bh.b_state = 0;
+			ret = (*dio->get_block)(inode, dio->block_in_file,
+						&map_bh, dio->rw == WRITE);
+			if (ret) {
+				printk("%s: get_block returns %d\n",
+					__FUNCTION__, ret);
+				goto fail_release;
+			}
+			/* blockdevs do not set buffer_new */
+			if (buffer_new(&map_bh))
+				unmap_underlying_metadata(map_bh.b_bdev,
+							map_bh.b_blocknr);
+			if (!buffer_mapped(&map_bh)) {
+				ret = -EINVAL;		/* A hole */
+				goto fail_release;
+			}
+			if (dio->bio) {
+				if (dio->bio->bi_idx == dio->bio->bi_vcnt ||
+						dio->boundary ||
+						dio->last_block_in_bio !=
+							map_bh.b_blocknr - 1) {
+					dio_bio_submit(dio);
+					dio->boundary = 0;
+				}
+			}
+			if (dio->bio == NULL) {
+				ret = dio_bio_reap(dio);
+				if (ret)
+					goto fail_release;
+				ret = dio_bio_alloc(dio, map_bh.b_bdev,
+					map_bh.b_blocknr << (blkbits - 9),
+					DIO_BIO_MAX_SIZE / PAGE_SIZE);
+				if (ret)
+					goto fail_release;
+				new_page = 1;
+				dio->boundary = 0;
+			}
+
+			bio = dio->bio;
+			if (new_page) {
+				dio->bvec = &bio->bi_io_vec[bio->bi_idx];
+				page_cache_get(page);
+				dio->bvec->bv_page = page;
+				dio->bvec->bv_len = 0;
+				dio->bvec->bv_offset = block_in_page*blocksize;
+				bio->bi_idx++;
+			}
+			new_page = 0;
+			dio->bvec->bv_len += blocksize;
+			bio->bi_size += blocksize;
+			dio->last_block_in_bio = map_bh.b_blocknr;
+			dio->boundary = buffer_boundary(&map_bh);
+
+			dio->block_in_file++;
+			if (dio->block_in_file >= dio->final_block_in_request)
+				break;
+		}
+		block_in_page = 0;
+		page_cache_release(page);
+	}
+	ret = 0;
+	goto out;
+fail_release:
+	page_cache_release(page);
+out:
+	return ret;
+}
+
+struct dio *g_dio;
+
+int
+generic_direct_IO(int rw, struct inode *inode, char *buf, loff_t offset,
+			size_t count, get_block_t get_block)
+{
+	const unsigned blocksize_mask = (1 << inode->i_blkbits) - 1;
+	const unsigned long user_addr = (unsigned long)buf;
+	int ret = 0;
+	int ret2;
+	struct dio dio;
+	size_t bytes;
+
+	/* Check the memory alignment.  Blocks cannot straddle pages */
+	if ((user_addr & blocksize_mask) || (count & blocksize_mask)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	g_dio = &dio;
+
+	/* BIO submission state */
+	dio.bio = NULL;
+	dio.bvec = NULL;
+	dio.inode = inode;
+	dio.rw = rw;
+	dio.block_in_file = offset >> inode->i_blkbits;
+	dio.final_block_in_request = (offset + count) >> inode->i_blkbits;
+
+	/* Index into the first page of the first block */
+	dio.first_block_in_page = (user_addr & (PAGE_SIZE - 1))
+						>> inode->i_blkbits;
+	dio.boundary = 0;
+	dio.reap_counter = 0;
+	dio.get_block = get_block;
+	dio.last_block_in_bio = -1;
+
+	/* Page fetching state */
+	dio.curr_page = 0;
+	bytes = count;
+	dio.total_pages = 0;
+	if (offset & PAGE_SIZE) {
+		dio.total_pages++;
+		bytes -= PAGE_SIZE - (offset & ~(PAGE_SIZE - 1));
+	}
+	dio.total_pages += (bytes + PAGE_SIZE - 1) / PAGE_SIZE;
+	dio.curr_user_address = user_addr;
+
+	/* Page queue */
+	dio.head = 0;
+	dio.tail = 0;
+
+	/* BIO completion state */
+	atomic_set(&dio.bio_count, 0);
+	spin_lock_init(&dio.bio_list_lock);
+	dio.bio_list = NULL;
+	init_waitqueue_head(&dio.wait_q);
+
+	down_read(&current->mm->mmap_sem);
+	ret = do_direct_IO(&dio);
+	up_read(&current->mm->mmap_sem);
+
+	if (dio.bio)
+		dio_bio_submit(&dio);
+	if (ret)
+		dio_cleanup(&dio);
+	ret2 = dio_await_completion(&dio);
+	if (ret == 0)
+		ret = ret2;
+	if (ret == 0)
+		ret = count - ((dio.final_block_in_request -
+				dio.block_in_file) << inode->i_blkbits);
+out:
+	return ret;
+}
+
+ssize_t
+generic_file_direct_IO(int rw, struct inode *inode, char *buf,
+			loff_t offset, size_t count)
+{
+	struct address_space *mapping = inode->i_mapping;
+	unsigned blocksize_mask;
+	ssize_t retval;
+
+	blocksize_mask = (1 << inode->i_blkbits) - 1;
+	if ((offset & blocksize_mask) || (count & blocksize_mask)) {
+		retval = -EINVAL;
+		goto out;
+	}
+
+	if (mapping->nrpages) {
+		retval = filemap_fdatawrite(mapping);
+		if (retval == 0)
+			retval = filemap_fdatawait(mapping);
+		if (retval)
+			goto out;
+	}
+	retval = mapping->a_ops->direct_IO(rw, inode, buf, offset, count);
+out:
+	return retval;
+}
diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
index b6678139657b..798875bd134e 100644
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -607,11 +607,10 @@ static int ext2_bmap(struct address_space *mapping, long block)
 }
 
 static int
-ext2_direct_IO(int rw, struct inode *inode, struct kiobuf *iobuf,
-			unsigned long blocknr, int blocksize)
+ext2_direct_IO(int rw, struct inode *inode, char *buf,
+			loff_t offset, size_t count)
 {
-	return generic_direct_IO(rw, inode, iobuf, blocknr,
-				blocksize, ext2_get_block);
+	return generic_direct_IO(rw, inode, buf, offset, count, ext2_get_block);
 }
 
 static int