Hi Jens and guys in block/io_uring mailing list, this mail contains some
discussion abount
RWF_NOWAIT, please see the following contents.
On 11/11/20 11:38 PM, Mike Snitzer wrote:
On Tue, Nov 10 2020 at 1:55am -0500,
Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx> wrote:
This is one prep patch for supporting iopoll for dm device.
The direct IO routine will set REQ_NOWAIT flag for REQ_HIPRI IO (that
is, IO will do iopoll) in bio_set_polled(). Then in the IO submission
routine, the ability of handling REQ_NOWAIT of the block device will
be checked for REQ_HIPRI IO in submit_bio_checks(). -EOPNOTSUPP will
be returned if the block device doesn't support REQ_NOWAIT.
submit_bio_checks() verifies the request_queue has QUEUE_FLAG_NOWAIT set
if the bio has REQ_NOWAIT.
Yes that's the case.
DM lacks support for REQ_NOWAIT until commit 6abc49468eea ("dm: add
support for REQ_NOWAIT and enable it for linear target"). Since then,
dm targets that support REQ_NOWAIT should advertise DM_TARGET_NOWAIT
feature.
I'm not seeing why DM_TARGET_NOWAIT is needed (since you didn't add any
code that consumes the flag).
As I said, it's needed if we support iopoll for dm device. Only if a
block device is capable of
handling NOWAIT, then it can support iopoll.
IO submitted for iopoll (marked with IOCB_HIPRI) is usually also marked
with REQ_NOWAIT.
There are two scenario when it could happen.
1. io_uring will set REQ_NOWAIT
The IO submission of io_uring can be divided into two phase. First, IO
will be submitted
synchronously in user process context (when sqthread feature disabled),
or sqthread
context (when sqthread feature enabled).
```sh
- current process context when sqthread disabled, or sqthread when it's
enabled
io_uring_enter
io_submit_sqes
io_submit_sqe
io_queue_sqe
__io_queue_sqe
io_issue_sqe // with @force_nonblock is true
io_read/io_write
```
In this case, IO should be handled in a NOWAIT way, since the user
process or sqthread
can not be blocked for performance.
```
io_read/io_write
/* Ensure we clear previously set non-block flag */
if (!force_nonblock)
kiocb->ki_flags &= ~IOCB_NOWAIT;
else
kiocb->ki_flags |= IOCB_NOWAIT;
```
2. The direct IO routine will set REQ_NOWAIT for polling IO
Both fs/block_dev.c: __blkdev_direct_IO and fs/iomap/direct-io.c:
iomap_dio_submit_bio will
call bio_set_polled(), in which will set REQ_NOWAIT for polling IO.
```sh
__blkdev_direct_IO / iomap_dio_submit_bio:
if (dio->iocb->ki_flags & IOCB_HIPRI)
bio_set_polled
bio->bi_opf |= REQ_NOWAIT
```
Thus to support iopoll for dm device, the dm target should be capable of
handling NOWAIT,
or submit_bio_checks() will fail with -EOPNOTSUPP when submitting bio to
dm device.
dm-table.c:dm_table_set_restrictions() has:
if (dm_table_supports_nowait(t))
blk_queue_flag_set(QUEUE_FLAG_NOWAIT, q);
else
blk_queue_flag_clear(QUEUE_FLAG_NOWAIT, q);
This patch adds support for DM_TARGET_NOWAIT for those dm targets, the
.map() algorithm of which just involves sector recalculation.
So you're looking to constrain which targets will properly support
REQ_NOWAIT, based on whether they do a simple remapping?
To be honest, I'm a little confused about the semantics of REQ_NOWAIT.
Jens may had ever
explained it in block or io_uring mailing list, but I can't find the
specific mail.
The man page explains FMODE_NOWAIT as 'File is capable of returning
-EAGAIN if I/O will
block'.
And RWF_NOWAIT as
```
RWF_NOWAIT (since Linux 4.14)
Don't wait if the I/O will block for operations
such as
file block allocations, dirty page flush, mutex locks,
or a congested block device inside the kernel. If any
of these conditions are met, the control block is re‐
turned immediately with a return value of -EAGAIN in
the res field of the io_event structure (see
io_getevents(2)).
```
commit 6abc49468eea ("dm: add support for REQ_NOWAIT and enable it for
linear
target") handles NOWAIT for DM core as
```
@@ -1802,7 +1802,9 @@ static blk_qc_t dm_submit_bio(struct bio *bio)
if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags))) {
+ if (bio->bi_opf & REQ_NOWAIT)
+ bio_wouldblock_error(bio);
+ else if (!(bio->bi_opf & REQ_RAHEAD))
queue_io(md, bio);
```
Theoretically the block device could advertise QUEUE_FLAG_NOWAIT as long
as it could
'return -EAGAIN if I/O will block' as the man page said. However,
considering when the
dm device detected as suspending, the submitted bios are deferred to
workqueue in
drivers/dm/dm.c: dm_submit_bio. In this case, IO gets **deferred** while
the user process
will not be **blocked**. Can we say IO gets **blocked** in this case?
Actually several dm targets handle submitted bio in this deferred way,
such as dm-crypt/
dm-delay/dm-era/dm-ebs. Can we say these targets are not capable of
handling NOWAIT?
Also when system is short of memory, bio allocation in
bio_alloc_bioset() may trigger memory
direct reclaim, as the gfp_mask is usually GFP_NOIO. While in memory
direct reclaim, the
process may be scheduled out, but I have never seen the proper handling
for NOWAIT in this
situation. Maybe the block or io_uring guys have more insights?
So there's just too many possibilities that may get blocked, not to say
mutex locks.
Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
---
Hi Mike,
I could split these boilerplate code that each dm target have one
seperate patch if you think that would be better.
One patch for all these is fine. But it should include the code that I
assume you'll be adding to dm_table_supports_nowait() to further verify
that the targets in the table are all DM_TARGET_NOWAIT.
And why isn't dm-linear setting DM_TARGET_NOWAIT?
These are all done in commit 6abc49468eea ("dm: add support for
REQ_NOWAIT and enable it for
linear target").
Also, other targets _could_ be made to support REQ_NOWAIT by
conditionally returning bio_wouldblock_error() if appropriate
(e.g. bio-based dm-multipath's case of queue_if_no_path).
--
Thanks,
Jeffle
