On Fri, Nov 11, 2016 at 10:11:25PM -0700, Jens Axboe wrote:
> From: Christoph Hellwig <hch@xxxxxx>
>
> 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@xxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxx>
> ---
> fs/block_dev.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 80 insertions(+)
>
[snip]
> 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);
__blockdev_direct_IO() does a few cache prefetches that we're now
bypassing, do we want to do the same in __blkdev_direct_IO_simple()?
That's the stuff added in 65dd2aa90aa1 ("dio: optimize cache misses in
the submission path").
> --
> 2.7.4
>
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--
Omar
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