On 11/14/2016 12:33 PM, Omar Sandoval wrote:
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").
Prefetches like that tend to grow stale, in my experience. So we should
probably just evaluate the new path cache behavior and see if it makes
sense.
--
Jens Axboe
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