loop: respect REQ_NOWAIT for memory allocation #376
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loop advertises REQ_NOWAIT support via BLK_FEAT_NOWAIT (set by
default for all blk-mq devices), but violates REQ_NOWAIT semantics
by using GFP_NOIO allocations which can block.
BLK_FEAT_NOWAIT is advertised through this call chain:
loop_add()
blk_mq_alloc_disk(&lim, ...)
__blk_mq_alloc_disk()
blk_mq_alloc_queue()
lim->features |= BLK_FEAT_NOWAIT <-- Set by default for blk-mq
However, the REQ_NOWAIT I/O path violates this contract. For io_uring
inline submissions, the call chain is:
1. Userspace (io_uring setup):
io_uring_setup(entries, ¶ms)
fd = io_uring_register(IORING_REGISTER_FILES, &zloop_fd, 1)
2. Userspace (submit I/O with NOWAIT):
sqe = io_uring_get_sqe(ring)
io_uring_prep_write(sqe, zloop_fd, buf, len, offset)
io_uring_submit(ring) <-- inline submission
3. io_uring core (inline path):
io_submit_sqes()
io_submit_sqe()
io_queue_sqe()
issue_flags = IO_URING_F_NONBLOCK <-- Sets inline/nowait mode
io_issue_sqe()
io_write()
if (force_nonblock) <-- true for inline submission
kiocb->ki_flags |= IOCB_NOWAIT
4. VFS layer:
call_write_iter()
blkdev_write_iter()
-> propagates IOCB_NOWAIT to REQ_NOWAIT on bio
__blkdev_direct_IO_simple() OR
__blkdev_direct_IO() OR
__blkdev_direct_IO_async()
5. Block layer:
blk_mq_submit_bio()
blk_mq_get_new_requests()
__blk_mq_alloc_requests()
blk_mq_rq_ctx_init()
-> propagates REQ_NOWAIT to request->cmd_flags
__blk_mq_try_issue_directly() OR blk_mq_sched_insert_request()
blk_mq_run_hw_queue()
__blk_mq_delay_run_hw_queue()
__blk_mq_run_hw_queue()
blk_mq_sched_dispatch_requests()
6. Loop driver:
loop_queue_rq()
lo_rw_aio()
kmalloc_array(..., GFP_NOIO) <-- BLOCKS (REQ_NOWAIT violation)
-> Should use GFP_NOWAIT when rq->cmd_flags & REQ_NOWAIT
Fix by using GFP_NOWAIT when REQ_NOWAIT is set, and return -EAGAIN
instead of -ENOMEM when NOWAIT allocation fails, allowing io_uring
to retry the operation with blocking allowed.
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
zloop advertises REQ_NOWAIT support via BLK_FEAT_NOWAIT (set by
default for all blk-mq devices), but violates REQ_NOWAIT semantics
by using GFP_NOIO allocations which can block.
BLK_FEAT_NOWAIT is advertised through this call chain:
zloop_add()
blk_mq_alloc_disk(&lim, ...)
__blk_mq_alloc_disk()
blk_mq_alloc_queue()
lim->features |= BLK_FEAT_NOWAIT <-- Set by default for blk-mq
However, the REQ_NOWAIT I/O path violates this contract. For io_uring
inline submissions, the call chain is:
1. Userspace (io_uring setup):
io_uring_setup(entries, ¶ms)
fd = io_uring_register(IORING_REGISTER_FILES, &zloop_fd, 1)
2. Userspace (submit I/O with NOWAIT):
sqe = io_uring_get_sqe(ring)
io_uring_prep_write(sqe, zloop_fd, buf, len, offset)
io_uring_submit(ring) <-- inline submission
3. io_uring core (inline path):
io_submit_sqes()
io_submit_sqe()
io_queue_sqe()
issue_flags = IO_URING_F_NONBLOCK <-- Sets inline/nowait mode
io_issue_sqe()
io_write()
if (force_nonblock) <-- true for inline submission
kiocb->ki_flags |= IOCB_NOWAIT
4. VFS layer:
call_write_iter()
blkdev_write_iter()
-> propagates IOCB_NOWAIT to REQ_NOWAIT on bio
__blkdev_direct_IO_simple() OR
__blkdev_direct_IO() OR
__blkdev_direct_IO_async()
5. Block layer:
blk_mq_submit_bio()
blk_mq_get_new_requests()
__blk_mq_alloc_requests()
blk_mq_rq_ctx_init()
-> propagates REQ_NOWAIT to request->cmd_flags
__blk_mq_try_issue_directly() OR blk_mq_sched_insert_request()
blk_mq_run_hw_queue()
__blk_mq_delay_run_hw_queue()
__blk_mq_run_hw_queue()
blk_mq_sched_dispatch_requests()
6. Zloop driver:
zloop_queue_rq()
zloop_rw()
kmalloc_array(..., GFP_NOIO) <-- BLOCKS (REQ_NOWAIT violation)
-> Should use GFP_NOWAIT when rq->cmd_flags & REQ_NOWAIT
Fix by using GFP_NOWAIT when REQ_NOWAIT is set, and return -EAGAIN
instead of -ENOMEM when NOWAIT allocation fails, allowing io_uring
to retry the operation with blocking allowed.
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
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Pull request for series with
subject: loop: respect REQ_NOWAIT for memory allocation
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1023918