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Date: Mon, 31 Oct 2016 11:59:25 -0600
From: Christoph Hellwig <hch@....de>
To: axboe@...com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] block: fast-path for small and simple direct I/O requests
This patch adds a small and simple fast patch for small direct I/O
requests on block devices that don't use AIO. Between the neat
bio_iov_iter_get_pages helper that avoids allocating a page array
for get_user_pages and the on-stack bio and biovec this avoid memory
allocations and atomic operations entirely in the direct I/O code
(lower levels might still do memory allocations and will usually
have at least some atomic operations, though).
Signed-off-by: Christoph Hellwig <hch@....de>
---
fs/block_dev.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 80 insertions(+)
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 05b5533..d4134a3 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -30,6 +30,7 @@
#include <linux/cleancache.h>
#include <linux/dax.h>
#include <linux/badblocks.h>
+#include <linux/task_io_accounting_ops.h>
#include <linux/falloc.h>
#include <asm/uaccess.h>
#include "internal.h"
@@ -175,12 +176,91 @@ static struct inode *bdev_file_inode(struct file *file)
return file->f_mapping->host;
}
+#define DIO_INLINE_BIO_VECS 4
+
+static void blkdev_bio_end_io_simple(struct bio *bio)
+{
+ struct task_struct *waiter = bio->bi_private;
+
+ WRITE_ONCE(bio->bi_private, NULL);
+ wake_up_process(waiter);
+}
+
+static ssize_t
+__blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
+ int nr_pages)
+{
+ struct file *file = iocb->ki_filp;
+ struct block_device *bdev = I_BDEV(bdev_file_inode(file));
+ unsigned blkbits = blksize_bits(bdev_logical_block_size(bdev));
+ struct bio_vec inline_vecs[DIO_INLINE_BIO_VECS], *bvec;
+ loff_t pos = iocb->ki_pos;
+ bool should_dirty = false;
+ struct bio bio;
+ ssize_t ret;
+ blk_qc_t qc;
+ int i;
+
+ if ((pos | iov_iter_alignment(iter)) & ((1 << blkbits) - 1))
+ return -EINVAL;
+
+ bio_init(&bio);
+ bio.bi_max_vecs = nr_pages;
+ bio.bi_io_vec = inline_vecs;
+ bio.bi_bdev = bdev;
+ bio.bi_iter.bi_sector = pos >> blkbits;
+ bio.bi_private = current;
+ bio.bi_end_io = blkdev_bio_end_io_simple;
+
+ ret = bio_iov_iter_get_pages(&bio, iter);
+ if (unlikely(ret))
+ return ret;
+ ret = bio.bi_iter.bi_size;
+
+ if (iov_iter_rw(iter) == READ) {
+ bio_set_op_attrs(&bio, REQ_OP_READ, 0);
+ if (iter->type == ITER_IOVEC)
+ should_dirty = true;
+ } else {
+ bio_set_op_attrs(&bio, REQ_OP_WRITE, WRITE_ODIRECT);
+ task_io_account_write(ret);
+ }
+
+ qc = submit_bio(&bio);
+ for (;;) {
+ set_current_state(TASK_UNINTERRUPTIBLE);
+ if (!READ_ONCE(bio.bi_private))
+ break;
+ if (!(iocb->ki_flags & IOCB_HIPRI) ||
+ !blk_poll(bdev_get_queue(bdev), qc))
+ io_schedule();
+ }
+ __set_current_state(TASK_RUNNING);
+
+ bio_for_each_segment_all(bvec, &bio, i) {
+ if (should_dirty && !PageCompound(bvec->bv_page))
+ set_page_dirty_lock(bvec->bv_page);
+ put_page(bvec->bv_page);
+ }
+
+ if (unlikely(bio.bi_error))
+ return bio.bi_error;
+ iocb->ki_pos += ret;
+ return ret;
+}
+
static ssize_t
blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
{
struct file *file = iocb->ki_filp;
struct inode *inode = bdev_file_inode(file);
+ int nr_pages;
+ nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
+ if (!nr_pages)
+ return 0;
+ if (is_sync_kiocb(iocb) && nr_pages <= DIO_INLINE_BIO_VECS)
+ return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
return __blockdev_direct_IO(iocb, inode, I_BDEV(inode), iter,
blkdev_get_block, NULL, NULL,
DIO_SKIP_DIO_COUNT);
--
2.1.4
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