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Subject[RFC PATCH v2 15/21] loop: use aio to perform io on the underlying file
Date
From: Zach Brown <zab@zabbo.net>

This uses the new kernel aio interface to process loopback IO by
submitting concurrent direct aio. Previously loop's IO was serialized
by synchronous processing in a thread.

The aio operations specify the memory for the IO with the bio_vec arrays
directly instead of mappings of the pages.

The use of aio operations is enabled when the backing file supports the
read_iter and write_iter methods. These methods must only be added when
O_DIRECT on bio_vecs is supported.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Zach Brown <zab@zabbo.net>
---
drivers/block/loop.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++-
include/linux/loop.h | 1 +
2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index cd50435..cdc34e1 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -76,6 +76,7 @@
#include <linux/sysfs.h>
#include <linux/miscdevice.h>
#include <linux/falloc.h>
+#include <linux/aio.h>

#include <asm/uaccess.h>

@@ -213,6 +214,46 @@ lo_do_transfer(struct loop_device *lo, int cmd,
return lo->transfer(lo, cmd, rpage, roffs, lpage, loffs, size, rblock);
}

+void lo_rw_aio_complete(u64 data, long res)
+{
+ struct bio *bio = (struct bio *)data;
+
+ if (res > 0)
+ res = 0;
+ else if (res < 0)
+ res = -EIO;
+
+ bio_endio(bio, res);
+}
+
+static int lo_rw_aio(struct loop_device *lo, struct bio *bio)
+{
+ struct file *file = lo->lo_backing_file;
+ struct kiocb *iocb;
+ unsigned short op;
+ struct iov_iter iter;
+ struct bio_vec *bvec;
+ size_t nr_segs;
+ loff_t pos = ((loff_t) bio->bi_sector << 9) + lo->lo_offset;
+
+ iocb = aio_kernel_alloc(GFP_NOIO);
+ if (!iocb)
+ return -ENOMEM;
+
+ if (bio_rw(bio) & WRITE)
+ op = IOCB_CMD_WRITE_ITER;
+ else
+ op = IOCB_CMD_READ_ITER;
+
+ bvec = bio_iovec_idx(bio, bio->bi_idx);
+ nr_segs = bio_segments(bio);
+ iov_iter_init_bvec(&iter, bvec, nr_segs, bvec_length(bvec, nr_segs), 0);
+ aio_kernel_init_iter(iocb, file, op, &iter, pos);
+ aio_kernel_init_callback(iocb, lo_rw_aio_complete, (u64)bio);
+
+ return aio_kernel_submit(iocb);
+}
+
/**
* __do_lo_send_write - helper for writing data to a loop device
*
@@ -512,7 +553,14 @@ static inline void loop_handle_bio(struct loop_device *lo, struct bio *bio)
do_loop_switch(lo, bio->bi_private);
bio_put(bio);
} else {
- int ret = do_bio_filebacked(lo, bio);
+ int ret;
+ if (lo->lo_flags & LO_FLAGS_USE_AIO &&
+ lo->transfer == transfer_none) {
+ ret = lo_rw_aio(lo, bio);
+ if (ret == 0)
+ return;
+ } else
+ ret = do_bio_filebacked(lo, bio);
bio_endio(bio, ret);
}
}
@@ -854,6 +902,11 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
!file->f_op->write)
lo_flags |= LO_FLAGS_READ_ONLY;

+ if (file->f_op->write_iter && file->f_op->read_iter) {
+ file->f_flags |= O_DIRECT;
+ lo_flags |= LO_FLAGS_USE_AIO;
+ }
+
lo_blocksize = S_ISBLK(inode->i_mode) ?
inode->i_bdev->bd_block_size : PAGE_SIZE;

diff --git a/include/linux/loop.h b/include/linux/loop.h
index 11a41a8..5163fd3 100644
--- a/include/linux/loop.h
+++ b/include/linux/loop.h
@@ -75,6 +75,7 @@ enum {
LO_FLAGS_READ_ONLY = 1,
LO_FLAGS_AUTOCLEAR = 4,
LO_FLAGS_PARTSCAN = 8,
+ LO_FLAGS_USE_AIO = 16,
};

#include <asm/posix_types.h> /* for __kernel_old_dev_t */
--
1.7.9.5


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