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[70.175.132.216]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-11b86afe12esm18537183c88.6.2025.11.15.18.52.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 15 Nov 2025 18:52:33 -0800 (PST) From: Chaitanya Kulkarni To: linux-block@vger.kernel.org Cc: axboe@kernel.dk, hch@lst.de, kch@nvidia.com, dlemoal@kernel.org, Chaitanya Kulkarni Subject: [PATCH 1/2] loop: respect REQ_NOWAIT for memory allocation Date: Sat, 15 Nov 2025 18:52:28 -0800 Message-Id: <20251116025229.29136-1-ckulkarnilinux@gmail.com> X-Mailer: git-send-email 2.40.0 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 --- drivers/block/loop.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 13ce229d450c..bad61f41df34 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -350,6 +350,8 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd, unsigned int offset; int nr_bvec = 0; int ret; + bool nowait = rq->cmd_flags & REQ_NOWAIT; + gfp_t alloc_flags = nowait ? GFP_NOWAIT : GFP_NOIO; rq_for_each_bvec(tmp, rq, rq_iter) nr_bvec++; @@ -357,9 +359,9 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd, if (rq->bio != rq->biotail) { bvec = kmalloc_array(nr_bvec, sizeof(struct bio_vec), - GFP_NOIO); + alloc_flags); if (!bvec) - return -EIO; + return nowait ? -EAGAIN : -ENOMEM; cmd->bvec = bvec; /* -- 2.40.0