[PATCH 2/2] zloop: respect REQ_NOWAIT for memory allocation

[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, &params)
   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>
---
 drivers/block/zloop.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zloop.c b/drivers/block/zloop.c
index 92be9f0af00a..9295c4817bd4 100644
--- a/drivers/block/zloop.c
+++ b/drivers/block/zloop.c
@@ -372,6 +372,8 @@ static void zloop_rw(struct zloop_cmd *cmd)
 	sector_t zone_end;
 	int nr_bvec = 0;
 	int ret;
+	bool nowait = rq->cmd_flags & REQ_NOWAIT;
+	gfp_t alloc_flags = nowait ? GFP_NOWAIT : GFP_NOIO;
 
 	atomic_set(&cmd->ref, 2);
 	cmd->sector = sector;
@@ -443,9 +445,9 @@ static void zloop_rw(struct zloop_cmd *cmd)
 	if (rq->bio != rq->biotail) {
 		struct bio_vec *bvec;
 
-		cmd->bvec = kmalloc_array(nr_bvec, sizeof(*cmd->bvec), GFP_NOIO);
+		cmd->bvec = kmalloc_array(nr_bvec, sizeof(*cmd->bvec), alloc_flags);
 		if (!cmd->bvec) {
-			ret = -EIO;
+			ret = nowait ? -EAGAIN : -ENOMEM;
 			goto unlock;
 		}
 
-- 
2.40.0