[PATCH v5 4/4] iomap: add simple dio path for small direct I/O
From: Fengnan Chang
Date: Mon Jun 29 2026 - 08:05:10 EST
When running 4K random read workloads on high-performance Gen5 NVMe
SSDs, the software overhead in the iomap direct I/O path
(__iomap_dio_rw) becomes a significant bottleneck.
Using io_uring with poll mode for a 4K randread test on a raw block
device:
taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
-n1 -P1 /dev/nvme10n1
Result: ~3.2M IOPS
Running the exact same workload on ext4 and XFS:
taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1
-n1 -P1 /mnt/testfile
Result: ~1.92M IOPS
Profiling the ext4 workload reveals that a significant portion of CPU
time is spent on memory allocation and the iomap state machine
iteration:
5.33% [kernel] [k] __iomap_dio_rw
3.26% [kernel] [k] iomap_iter
2.37% [kernel] [k] iomap_dio_bio_iter
2.35% [kernel] [k] kfree
1.33% [kernel] [k] iomap_dio_complete
Introduce a simple dio path to reduce the overhead of iomap. It is
triggered when the request satisfies all of:
- a READ request whose I/O size is <= inode blocksize (fits in a single
block, no splits);
- no custom iomap_dio_ops (dops) registered by the filesystem;
- no caller-accumulated residual (done_before == 0);
- none of IOMAP_DIO_FORCE_WAIT / IOMAP_DIO_PARTIAL / IOMAP_DIO_BOUNCE
set, the range is within i_size, and the inode is not encrypted.
The bio is allocated from a dedicated bioset whose front_pad embeds
struct iomap_dio_simple, so the whole request lives in a single
cacheline-aligned allocation and no separate struct iomap_dio is
needed. Completion is handled inline from ->bi_end_io for the common
success case, and only punted to the s_dio_done_wq workqueue on error.
After this optimization, the heavy generic functions disappear from the
profile, replaced by a single streamlined execution path:
4.83% [kernel] [k] iomap_dio_simple
With this patch, 4K random read IOPS on ext4 increases from 1.92M to
2.19M in the original single-core io_uring poll-mode workload.
Below are the fio 4K direct randread results from a broader QD
sweep on a SOLIDIGM SB5PH27X038T NVMe SSD. Each result is the
average of three 20-second runs after a 5-second ramp time, using a
128G test file and toggling /sys/kernel/iomap/simple_read:
fs workload qd simple=0 simple=1 gain
ext4 libaio 1 18,740 18,761 +0.11%
ext4 libaio 64 462,850 480,587 +3.83%
ext4 libaio 128 459,498 478,824 +4.21%
ext4 libaio 256 459,938 480,156 +4.40%
ext4 io_uring 1 18,836 18,880 +0.24%
ext4 io_uring 64 568,193 600,625 +5.71%
ext4 io_uring 128 570,998 602,148 +5.46%
ext4 io_uring 256 572,052 602,536 +5.33%
ext4 io_uring_poll 1 19,283 19,272 -0.06%
ext4 io_uring_poll 64 989,735 1,013,342 +2.39%
ext4 io_uring_poll 128 1,467,336 1,538,444 +4.85%
ext4 io_uring_poll 256 1,663,498 1,830,842 +10.06%
xfs libaio 1 18,764 18,776 +0.06%
xfs libaio 64 462,408 480,860 +3.99%
xfs libaio 128 461,280 480,819 +4.24%
xfs libaio 256 461,626 480,190 +4.02%
xfs io_uring 1 18,871 18,903 +0.17%
xfs io_uring 64 570,383 597,399 +4.74%
xfs io_uring 128 568,290 597,370 +5.12%
xfs io_uring 256 570,616 598,775 +4.93%
xfs io_uring_poll 1 19,211 19,315 +0.54%
xfs io_uring_poll 64 989,726 1,008,455 +1.89%
xfs io_uring_poll 128 1,430,426 1,513,064 +5.78%
xfs io_uring_poll 256 1,587,339 1,742,220 +9.76%
Signed-off-by: Fengnan Chang <changfengnan@xxxxxxxxxxxxx>
---
fs/iomap/direct-io.c | 252 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 252 insertions(+)
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 4db79ddc1ddd4..4dcc9fc766a92 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -10,6 +10,9 @@
#include <linux/iomap.h>
#include <linux/task_io_accounting_ops.h>
#include <linux/fserror.h>
+#include <linux/kobject.h>
+#include <linux/sysfs.h>
+#include <linux/init.h>
#include "internal.h"
#include "trace.h"
@@ -899,12 +902,253 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
}
EXPORT_SYMBOL_GPL(__iomap_dio_rw);
+struct iomap_dio_simple {
+ struct kiocb *iocb;
+ size_t size;
+ unsigned int dio_flags;
+ struct work_struct work;
+ /*
+ * Align @bio to a cacheline boundary so that, combined with the
+ * front_pad passed to bioset_init(), the bio sits at the start of
+ * a cacheline in memory returned by the (HWCACHE-aligned) bio
+ * slab. This keeps the hot fields block layer touches on submit
+ * and completion (bi_iter, bi_status, ...) within a single line.
+ */
+ struct bio bio ____cacheline_aligned_in_smp;
+};
+
+static struct bio_set iomap_dio_simple_pool;
+
+static ssize_t iomap_dio_simple_complete(struct iomap_dio_simple *sr)
+{
+ struct bio *bio = &sr->bio;
+ struct kiocb *iocb = sr->iocb;
+ struct inode *inode = file_inode(iocb->ki_filp);
+ int error = blk_status_to_errno(bio->bi_status);
+ ssize_t ret = error;
+
+ if (likely(!ret)) {
+ ret = sr->size;
+ iocb->ki_pos += ret;
+ } else if (should_report_dio_fserror(ret)) {
+ fserror_report_io(inode, FSERR_DIRECTIO_READ, iocb->ki_pos,
+ sr->size, ret, GFP_NOFS);
+ }
+
+ iomap_dio_bio_release_pages(bio, sr->dio_flags, ret < 0);
+ inode_dio_end(inode);
+ trace_iomap_dio_complete(iocb, error, ret);
+ return ret;
+}
+
+static void iomap_dio_simple_complete_work(struct work_struct *work)
+{
+ struct iomap_dio_simple *sr =
+ container_of(work, struct iomap_dio_simple, work);
+
+ WRITE_ONCE(sr->iocb->private, NULL);
+ sr->iocb->ki_complete(sr->iocb, iomap_dio_simple_complete(sr));
+}
+
+static void iomap_dio_simple_end_io(struct bio *bio)
+{
+ struct iomap_dio_simple *sr =
+ container_of(bio, struct iomap_dio_simple, bio);
+
+ if (unlikely(sr->bio.bi_status)) {
+ struct inode *inode = file_inode(sr->iocb->ki_filp);
+
+ INIT_WORK(&sr->work, iomap_dio_simple_complete_work);
+ queue_work(inode->i_sb->s_dio_done_wq, &sr->work);
+ return;
+ }
+
+ sr->iocb->ki_complete(sr->iocb, iomap_dio_simple_complete(sr));
+}
+
+static inline bool iomap_dio_simple_supported(struct kiocb *iocb,
+ struct iov_iter *iter, unsigned int dio_flags)
+{
+ struct inode *inode = file_inode(iocb->ki_filp);
+ size_t count = iov_iter_count(iter);
+
+ if (iov_iter_rw(iter) != READ)
+ return false;
+ if (!count)
+ return false;
+ /*
+ * Simple dio is an optimization for small IO. Filter out large IO
+ * early as it's the most common case to fail for typical direct IO
+ * workloads.
+ */
+ if (count > inode->i_sb->s_blocksize)
+ return false;
+ if (dio_flags & (IOMAP_DIO_FORCE_WAIT | IOMAP_DIO_PARTIAL |
+ IOMAP_DIO_BOUNCE))
+ return false;
+ if (iocb->ki_pos + count > i_size_read(inode))
+ return false;
+ if (IS_ENCRYPTED(inode))
+ return false;
+
+ return true;
+}
+
+static ssize_t iomap_dio_simple(struct kiocb *iocb,
+ struct iov_iter *iter, const struct iomap_ops *ops,
+ void *private, unsigned int dio_flags)
+{
+ struct inode *inode = file_inode(iocb->ki_filp);
+ size_t count = iov_iter_count(iter);
+ struct iomap_dio_simple *sr;
+ unsigned int alignment;
+ struct iomap_iter iomi = {
+ .inode = inode,
+ .pos = iocb->ki_pos,
+ .len = count,
+ .flags = IOMAP_DIRECT,
+ .private = private,
+ };
+ struct bio *bio;
+ bool wait_for_completion = is_sync_kiocb(iocb);
+ ssize_t ret;
+
+ if (iocb->ki_flags & IOCB_NOWAIT)
+ iomi.flags |= IOMAP_NOWAIT;
+
+ ret = kiocb_write_and_wait(iocb, count);
+ if (ret)
+ return ret;
+
+ inode_dio_begin(inode);
+
+ ret = ops->iomap_begin(inode, iomi.pos, count, iomi.flags,
+ &iomi.iomap, &iomi.srcmap);
+ if (ret) {
+ inode_dio_end(inode);
+ return ret;
+ }
+
+ if (iomi.iomap.type != IOMAP_MAPPED ||
+ iomi.iomap.offset + iomi.iomap.length < iomi.pos + count ||
+ (iomi.iomap.flags & IOMAP_F_INTEGRITY)) {
+ ret = -ENOTBLK;
+ goto out_iomap_end;
+ }
+
+ alignment = iomap_dio_alignment(inode, iomi.iomap.bdev, dio_flags);
+ if ((iomi.pos | count) & (alignment - 1)) {
+ ret = -EINVAL;
+ goto out_iomap_end;
+ }
+
+ if (!wait_for_completion && unlikely(!inode->i_sb->s_dio_done_wq)) {
+ ret = sb_init_dio_done_wq(inode->i_sb);
+ if (ret < 0)
+ goto out_iomap_end;
+ }
+
+ trace_iomap_dio_rw_begin(iocb, iter, dio_flags, 0);
+
+ if (user_backed_iter(iter))
+ dio_flags |= IOMAP_DIO_USER_BACKED;
+
+ bio = bio_alloc_bioset(iomi.iomap.bdev,
+ bio_iov_vecs_to_alloc(iter, BIO_MAX_VECS),
+ REQ_OP_READ,
+ GFP_KERNEL, &iomap_dio_simple_pool);
+ sr = container_of(bio, struct iomap_dio_simple, bio);
+ sr->iocb = iocb;
+ sr->dio_flags = dio_flags;
+
+ bio->bi_iter.bi_sector = iomap_sector(&iomi.iomap, iomi.pos);
+ bio->bi_ioprio = iocb->ki_ioprio;
+
+ ret = bio_iov_iter_get_pages(bio, iter, alignment - 1);
+ if (unlikely(ret))
+ goto out_bio_put;
+
+ if (bio->bi_iter.bi_size != count) {
+ iov_iter_revert(iter, bio->bi_iter.bi_size);
+ ret = -ENOTBLK;
+ goto out_bio_release_pages;
+ }
+
+ sr->size = bio->bi_iter.bi_size;
+
+ if (dio_flags & IOMAP_DIO_USER_BACKED)
+ bio_set_pages_dirty(bio);
+
+ if (iocb->ki_flags & IOCB_NOWAIT)
+ bio->bi_opf |= REQ_NOWAIT;
+ if ((iocb->ki_flags & IOCB_HIPRI) && !wait_for_completion) {
+ bio->bi_opf |= REQ_POLLED;
+ WRITE_ONCE(iocb->private, bio);
+ }
+
+ if (ops->iomap_end)
+ ops->iomap_end(inode, iomi.pos, count, count, iomi.flags,
+ &iomi.iomap);
+
+ if (!wait_for_completion) {
+ bio->bi_end_io = iomap_dio_simple_end_io;
+ submit_bio(bio);
+ trace_iomap_dio_rw_queued(inode, iomi.pos, count);
+ return -EIOCBQUEUED;
+ }
+
+ submit_bio_wait(bio);
+ return iomap_dio_simple_complete(sr);
+
+out_bio_release_pages:
+ bio_release_pages(bio, false);
+out_bio_put:
+ bio_put(bio);
+out_iomap_end:
+ if (ops->iomap_end)
+ ops->iomap_end(inode, iomi.pos, count, 0, iomi.flags,
+ &iomi.iomap);
+ inode_dio_end(inode);
+ return ret;
+}
+
ssize_t
iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
const struct iomap_ops *ops, const struct iomap_dio_ops *dops,
unsigned int dio_flags, void *private, size_t done_before)
{
struct iomap_dio *dio;
+ ssize_t ret;
+
+ /*
+ * Fast path for small, block-aligned direct I/Os that map to a
+ * single contiguous on-disk extent.
+ *
+ * @dops must be NULL: a non-NULL @dops means the caller wants its
+ * ->end_io / ->submit_io hooks invoked, and in particular wants its
+ * bios to be allocated from the filesystem-private @dops->bio_set
+ * (whose front_pad sizes a filesystem-private wrapper around the
+ * bio). The fast path instead allocates from the shared
+ * iomap_dio_simple_pool, whose front_pad matches
+ * struct iomap_dio_simple; the two wrappers are not
+ * interchangeable, so we must fall back to __iomap_dio_rw() in
+ * that case.
+ *
+ * @done_before must be zero: a non-zero caller-accumulated residual
+ * cannot be carried through a single-bio inline completion.
+ *
+ * -ENOTBLK is the private sentinel returned by iomap_dio_simple()
+ * when it decides the request does not fit the fast path.
+ * In that case we proceed to the generic __iomap_dio_rw() slow
+ * path. Any other errno is a real result and is propagated as-is,
+ * in particular -EAGAIN for IOCB_NOWAIT must reach the caller.
+ */
+ if (!dops && !done_before &&
+ iomap_dio_simple_supported(iocb, iter, dio_flags)) {
+ ret = iomap_dio_simple(iocb, iter, ops, private, dio_flags);
+ if (ret != -ENOTBLK)
+ return ret;
+ }
dio = __iomap_dio_rw(iocb, iter, ops, dops, dio_flags, private,
done_before);
@@ -913,3 +1157,11 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
return iomap_dio_complete(dio);
}
EXPORT_SYMBOL_GPL(iomap_dio_rw);
+
+static int __init iomap_dio_init(void)
+{
+ return bioset_init(&iomap_dio_simple_pool, 4,
+ offsetof(struct iomap_dio_simple, bio),
+ BIOSET_NEED_BVECS | BIOSET_PERCPU_CACHE);
+}
+fs_initcall(iomap_dio_init);
--
2.39.5 (Apple Git-154)