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SubjectRe: [PATCH 2/2] block: fast-path for small and simple direct I/O requests
On Mon, Oct 31, 2016 at 11:59:25AM -0600, Christoph Hellwig wrote:
> 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>
> ---
> 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)

Nit: iter_is_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);

The bio_iov_iter_get_pages() trick is nifty. Jens had a similarly
stripped-down version of blkdev_direct_IO() for blk-mq as part of his
blk-dio branch. Looks like he just had the page array on the stack but
still had to allocate the bio.

--
Omar

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