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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@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
Tested-By: Stephen Bates <sbates@raithlin.com>
Reviewed-By: Stephen Bates <sbates@raithlin.com>
This commit is contained in:
Christoph Hellwig 2016-10-31 11:59:25 -06:00 committed by Jens Axboe
parent 429a787be6
commit 189ce2b9dc
1 changed files with 80 additions and 0 deletions
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80
fs/block_dev.c
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@ -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_is_iovec(iter))
should_dirty = true;
} else {
bio_set_op_attrs(&bio, REQ_OP_WRITE, REQ_SYNC | REQ_IDLE);
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_mq_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);