[PATCH] nvme: provide fallback for discard alloc failure

[Jens Axboe <axboe@xxxxxxxxx>]

When boxes are run near (or to) OOM, we have a problem with the discard
page allocation in sd. If we fail allocating the special page, we return
busy, and it'll get retried. But since ordering is honored for dispatch
requests, we can keep retrying this same IO and failing. Behind that IO
could be requests that want to free memory, but they never get the
chance.

Allocate a fixed discard page per controller for a safe fallback, and use
that if the initial allocation fails.

Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

---

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index c71e879821ad..7ca988e58790 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -564,9 +564,14 @@ static blk_status_t nvme_setup_discard(struct nvme_ns *ns, struct request *req,
 	struct nvme_dsm_range *range;
 	struct bio *bio;
 
-	range = kmalloc_array(segments, sizeof(*range), GFP_ATOMIC);
-	if (!range)
-		return BLK_STS_RESOURCE;
+	range = kmalloc_array(segments, sizeof(*range),
+				GFP_ATOMIC | __GFP_NOWARN);
+	if (!range) {
+		if (test_and_set_bit_lock(0, &ns->ctrl->discard_page_busy))
+			return BLK_STS_RESOURCE;
+
+		range = page_address(ns->ctrl->discard_page);
+	}
 
 	__rq_for_each_bio(bio, req) {
 		u64 slba = nvme_block_nr(ns, bio->bi_iter.bi_sector);
@@ -581,7 +586,10 @@ static blk_status_t nvme_setup_discard(struct nvme_ns *ns, struct request *req,
 	}
 
 	if (WARN_ON_ONCE(n != segments)) {
-		kfree(range);
+		if (virt_to_page(range) == ns->ctrl->discard_page)
+			clear_bit_unlock(0, &ns->ctrl->discard_page_busy);
+		else
+			kfree(range);
 		return BLK_STS_IOERR;
 	}
 
@@ -664,8 +672,13 @@ void nvme_cleanup_cmd(struct request *req)
 				blk_rq_bytes(req) >> ns->lba_shift);
 	}
 	if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
-		kfree(page_address(req->special_vec.bv_page) +
-		      req->special_vec.bv_offset);
+		struct nvme_ns *ns = req->rq_disk->private_data;
+		struct page *page = req->special_vec.bv_page;
+
+		if (page == ns->ctrl->discard_page)
+			clear_bit_unlock(0, &ns->ctrl->discard_page_busy);
+		else
+			kfree(page_address(page) + req->special_vec.bv_offset);
 	}
 }
 EXPORT_SYMBOL_GPL(nvme_cleanup_cmd);
@@ -3578,6 +3591,7 @@ static void nvme_free_ctrl(struct device *dev)
 	ida_simple_remove(&nvme_instance_ida, ctrl->instance);
 	kfree(ctrl->effects);
 	nvme_mpath_uninit(ctrl);
+	kfree(ctrl->discard_page);
 
 	if (subsys) {
 		mutex_lock(&subsys->lock);
@@ -3618,6 +3632,12 @@ int nvme_init_ctrl(struct nvme_ctrl *ctrl, struct device *dev,
 	memset(&ctrl->ka_cmd, 0, sizeof(ctrl->ka_cmd));
 	ctrl->ka_cmd.common.opcode = nvme_admin_keep_alive;
 
+	ctrl->discard_page = alloc_page(GFP_KERNEL);
+	if (!ctrl->discard_page) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
 	ret = ida_simple_get(&nvme_instance_ida, 0, 0, GFP_KERNEL);
 	if (ret < 0)
 		goto out;
@@ -3655,6 +3675,8 @@ int nvme_init_ctrl(struct nvme_ctrl *ctrl, struct device *dev,
 out_release_instance:
 	ida_simple_remove(&nvme_instance_ida, ctrl->instance);
 out:
+	if (ctrl->discard_page)
+		__free_page(ctrl->discard_page);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(nvme_init_ctrl);
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index e20e737ac10c..f1fe88598a04 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -241,6 +241,9 @@ struct nvme_ctrl {
 	u16 maxcmd;
 	int nr_reconnects;
 	struct nvmf_ctrl_options *opts;
+
+	struct page *discard_page;
+	unsigned long discard_page_busy;
 };
 
 struct nvme_subsystem {

-- 
Jens Axboe