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SubjectRe: [patch] optimize o_direct on block device - v2
On Mon, 4 Dec 2006 20:55:50 -0800
"Chen, Kenneth W" <kenneth.w.chen@intel.com> wrote:

> This patch implements block device specific .direct_IO method instead
> of going through generic direct_io_worker for block device.
>
> direct_io_worker is fairly complex because it needs to handle O_DIRECT
> on file system, where it needs to perform block allocation, hole detection,
> extents file on write, and tons of other corner cases. The end result is
> that it takes tons of CPU time to submit an I/O.
>
> For block device, the block allocation is much simpler and a tight triple
> loop can be written to iterate each iovec and each page within the iovec
> in order to construct/prepare bio structure and then subsequently submit
> it to the block layer. This significantly speeds up O_D on block device.
>

Looks reasonable to me. A few musings:

>
> -static int
> -blkdev_get_blocks(struct inode *inode, sector_t iblock,
> - struct buffer_head *bh, int create)
> +int blk_end_aio(struct bio *bio, unsigned int bytes_done, int error)
> {
> - sector_t end_block = max_block(I_BDEV(inode));
> - unsigned long max_blocks = bh->b_size >> inode->i_blkbits;
> + struct kiocb* iocb = bio->bi_private;
> + atomic_t* bio_count = (atomic_t*) &iocb->private;

This atomic_t-in-a-void* thing is rather unpleasing.

It could be a new member of `struct kiocb', perhaps.

Please use " *" rather than "* ".

> + long res;
> +
> + if ((bio->bi_rw & 1) == READ)

bio_data_dir()

> + bio_check_pages_dirty(bio);
> + else {
> + bio_release_pages(bio);
> + bio_put(bio);
> + }
>
> - if ((iblock + max_blocks) > end_block) {
> - max_blocks = end_block - iblock;
> - if ((long)max_blocks <= 0) {
> - if (create)
> - return -EIO; /* write fully beyond EOF */
> - /*
> - * It is a read which is fully beyond EOF. We return
> - * a !buffer_mapped buffer
> - */
> - max_blocks = 0;
> - }
> + if (error)
> + iocb->ki_left = -EIO;
> +
> + if (atomic_dec_and_test(bio_count)) {
> + res = (iocb->ki_left < 0) ? iocb->ki_left : iocb->ki_nbytes;
> + aio_complete(iocb, res, 0);
> }
>
> - bh->b_bdev = I_BDEV(inode);
> - bh->b_blocknr = iblock;
> - bh->b_size = max_blocks << inode->i_blkbits;
> - if (max_blocks)
> - set_buffer_mapped(bh);
> return 0;
> }
>
> +#define VEC_SIZE 16
> +struct pvec {
> + unsigned short nr;
> + unsigned short idx;
> + struct page *page[VEC_SIZE];
> +};

Do we actually need this? afaict all it does is saves an additional
function arg in a couple of places.

I guess it doesn't hurt though.

> +
> +struct page *blk_get_page(unsigned long addr, size_t count, int rw,
> + struct pvec *pvec)

Does this need kernel-wide scope?

A nice introductory comment explaining what it does would be, err, nice.

> +{
> + int ret, nr_pages;
> + if (pvec->idx == pvec->nr) {
> + nr_pages = (addr + count + PAGE_SIZE - 1) / PAGE_SIZE -
> + addr / PAGE_SIZE;
> + nr_pages = min(nr_pages, VEC_SIZE);
> + down_read(&current->mm->mmap_sem);
> + ret = get_user_pages(current, current->mm, addr, nr_pages,
> + rw==READ, 0, pvec->page, NULL);
> + up_read(&current->mm->mmap_sem);
> + if (ret < 0)
> + return ERR_PTR(ret);
> + pvec->nr = ret;
> + pvec->idx = 0;
> + }
> + return pvec->page[pvec->idx++];
> +}

hm, if get_user_pages() returns zero, badness ensues.

It won't do that, unless perhaps we passed it a zero nr_pages in the first
place. We shouldn't do that.

Has this code been tested with zero-length writes? And with iovecs which
contain a zero-length segment?


> static ssize_t
> blkdev_direct_IO(int rw, struct kiocb *iocb, const struct iovec *iov,
> - loff_t offset, unsigned long nr_segs)
> + loff_t pos, unsigned long nr_segs)
> {
> - struct file *file = iocb->ki_filp;
> - struct inode *inode = file->f_mapping->host;
> + struct inode *inode = iocb->ki_filp->f_mapping->host;
> + unsigned blkbits = blksize_bits(bdev_hardsect_size(I_BDEV(inode)));
> + unsigned blocksize_mask = (1<< blkbits) - 1;
> + unsigned long seg, nvec, cur_off, cur_len;
> +
> + unsigned long addr;
> + size_t count, nbytes = iocb->ki_nbytes;
> + loff_t size;
> + struct bio * bio;
> + atomic_t *bio_count = (atomic_t *) &iocb->private;
> + struct page *page;
> + struct pvec pvec = {.nr = 0, .idx = 0, };

Please use one declaration per line (no commas). That leaves you room for
a little comment alongside each local, explaining its operation.

This function needs little comments alongside each local, explaining their
operation.

> + BUILD_BUG_ON(sizeof(atomic_t) > sizeof(iocb->private));

argh. And if it triggers?

> + size = i_size_read(inode);
> + if (pos + nbytes > size)
> + nbytes = size - pos;
> +
> + seg = 0;
> + addr = (unsigned long) iov[0].iov_base;
> + count = iov[0].iov_len;
> + atomic_set(bio_count, 1);
> +
> + /* first check the alignment */
> + if (addr & blocksize_mask || count & blocksize_mask ||
> + pos & blocksize_mask)
> + return -EINVAL;
> +
> + while (nbytes) {
> + /* roughly estimate number of bio vec needed */
> + nvec = (nbytes + PAGE_SIZE - 1) / PAGE_SIZE;
> + nvec = max(nvec, nr_segs - seg);
> + nvec = min(nvec, (unsigned long) BIO_MAX_PAGES);
> +
> + /* bio_alloc should not fail with GFP_KERNEL flag */
> + bio = bio_alloc(GFP_KERNEL, nvec);
> + bio->bi_bdev = I_BDEV(inode);
> + bio->bi_end_io = blk_end_aio;
> + bio->bi_private = iocb;
> + bio->bi_sector = pos >> blkbits;
> +same_bio:
> + cur_off = addr & ~PAGE_MASK;
> + cur_len = PAGE_SIZE - cur_off;
> + if (count < cur_len)
> + cur_len = count;
> +
> + page = blk_get_page(addr, count, rw, &pvec);
> + if (unlikely(IS_ERR(page)))
> + goto backout;
> +
> + if (bio_add_page(bio, page, cur_len, cur_off)) {
> + pos += cur_len;
> + addr += cur_len;
> + count -= cur_len;
> + nbytes -= cur_len;
> +
> + if (count)
> + goto same_bio;
> + if (++seg < nr_segs) {
> + addr = (unsigned long) iov[seg].iov_base;
> + count = iov[seg].iov_len;
> + if (unlikely(addr & blocksize_mask ||
> + count & blocksize_mask))
> + goto backout;
> + goto same_bio;
> + }
> + }
> +
> + /* bio is ready, submit it */
> + if (rw == READ)
> + bio_set_pages_dirty(bio);
> + atomic_inc(bio_count);
> + submit_bio(rw, bio);

This function can place a potentially unbounded amount of memory under IO.
hence it can permit the user to pin a potentially unbounded amount of memry
for potentially unbounded amounts of time. That "64" in dio_bio_reap() is
a lame attempt to limit this.

OK, so it's a privileged operation.

> + }
> +
> +completion:
> + nbytes = iocb->ki_nbytes = iocb->ki_nbytes - nbytes;

Kernel coding-style is super-simple-style. Please avoid tricky things and
just do

a = b;
c = b;


> + iocb->ki_pos += nbytes;
>
> - return blockdev_direct_IO_no_locking(rw, iocb, inode, I_BDEV(inode),
> - iov, offset, nr_segs, blkdev_get_blocks, NULL);
> + blk_run_address_space(inode->i_mapping);
> + if (atomic_dec_and_test(bio_count))
> + aio_complete(iocb, nbytes, 0);
> +
> + return -EIOCBQUEUED;
> +
> +backout:
> + /*
> + * back out nbytes count constructed so far for this bio,
> + * we will throw away current bio.
> + */
> + nbytes -= bio->bi_size;
> + bio_release_pages(bio);
> + bio_put(bio);
> +
> + /*
> + * if no bio was submmitted, return the error code.
> + * otherwise, proceed with pending I/O completion.
> + */
> + if (atomic_read(bio_count) == 1)
> + return PTR_ERR(page);
> + goto completion;
> }
>
> static int blkdev_writepage(struct page *page, struct writeback_control *wbc)
> --- ./fs/read_write.c.orig 2006-11-29 13:57:37.000000000 -0800
> +++ ./fs/read_write.c 2006-12-04 17:30:34.000000000 -0800
> @@ -235,7 +235,7 @@ ssize_t do_sync_read(struct file *filp,
>
> init_sync_kiocb(&kiocb, filp);
> kiocb.ki_pos = *ppos;
> - kiocb.ki_left = len;
> + kiocb.ki_nbytes = kiocb.ki_left = len;

What's this doing here?


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