[Devel] [PATCH vz9/vz10] dm-ploop: fallback to kvmalloc for large bvec allocations

[Vasileios Almpanis]

When handling multiple concurrent dm-ploop requests, large bio_vec arrays
can be allocated during request processing. These allocations are currently
done with kmalloc_array(GFP_ATOMIC), which can fail under memory pressure
for higher orders (order >= 6, ~256KB). Such failures result in partial or
corrupted I/O, leading to EXT4 directory checksum errors and read-only
remounts under heavy parallel workloads.

This patch adds a fallback mechanism to use kvmalloc_array for
large or failed allocations. If the estimated allocation order is >= 6, or
if the kmalloc_array allocation fails. This avoids high-order GFP_ATOMIC
allocations from interrupt context and ensures more reliable memory allocation
behavior.

https://virtuozzo.atlassian.net/browse/VSTOR-109595
Signed-off-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
Feature: dm-ploop: ploop target driver
---
 drivers/md/dm-ploop-map.c | 46 ++++++++++++++++++++++++++++++---------
 drivers/md/dm-ploop.h     |  1 +
 2 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/drivers/md/dm-ploop-map.c b/drivers/md/dm-ploop-map.c
index 3fb841f8bcea..899b9bf088b3 100644
--- a/drivers/md/dm-ploop-map.c
+++ b/drivers/md/dm-ploop-map.c
@@ -16,6 +16,7 @@
 #include <linux/error-injection.h>
 #include <linux/uio.h>
 #include <linux/blk-mq.h>
+#include <linux/mm.h>
 #include <uapi/linux/falloc.h>
 #include "dm-ploop.h"
 #include "dm-rq.h"
@@ -89,6 +90,7 @@ void ploop_init_pio(struct ploop *ploop, unsigned int bi_op, struct pio *pio)
 	pio->ref_index = PLOOP_REF_INDEX_INVALID;
 	pio->queue_list_id = PLOOP_LIST_DEFERRED;
 	pio->bi_status = BLK_STS_OK;
+	pio->use_kvmalloc = false;
 	atomic_set(&pio->remaining, 1);
 	pio->piwb = NULL;
 	INIT_LIST_HEAD(&pio->list);
@@ -193,8 +195,12 @@ static void ploop_prq_endio(struct pio *pio, void *prq_ptr,
 	struct ploop_rq *prq = prq_ptr;
 	struct request *rq = prq->rq;
 
-	if (prq->bvec)
-		kfree(prq->bvec);
+	if (prq->bvec) {
+		if (pio->use_kvmalloc)
+			kvfree(prq->bvec);
+		else
+			kfree(prq->bvec);
+	}
 	if (prq->css)
 		css_put(prq->css);
 	/*
@@ -1963,26 +1969,40 @@ void ploop_index_wb_submit(struct ploop *ploop, struct ploop_index_wb *piwb)
 	ploop_runners_add_work(ploop, pio);
 }
 
-static struct bio_vec *ploop_create_bvec_from_rq(struct request *rq)
+static struct bio_vec *ploop_create_bvec_from_rq(struct request *rq, bool use_kvmalloc)
 {
 	struct bio_vec bv, *bvec, *tmp;
 	struct req_iterator rq_iter;
 	unsigned int nr_bvec = 0;
+	unsigned int order = 0;
 
 	rq_for_each_bvec(bv, rq, rq_iter)
 		nr_bvec++;
 
-	bvec = kmalloc_array(nr_bvec, sizeof(struct bio_vec),
-			     GFP_ATOMIC);
-	if (!bvec)
-		goto out;
+	if (use_kvmalloc) {
+		bvec = kvmalloc_array(nr_bvec, sizeof(struct bio_vec),
+				      GFP_NOIO);
+		if (!bvec)
+			return ERR_PTR(-ENOMEM);
+	} else {
+		order = get_order(nr_bvec * sizeof(struct bio_vec));
+		/*
+		 * order 6 is 262144 bytes. Lets defer such big
+		 * allocations to workqueue.
+		 */
+		if (order >= 6)
+			return ERR_PTR(-EAGAIN);
+		bvec = kmalloc_array(nr_bvec, sizeof(struct bio_vec),
+				     GFP_ATOMIC | __GFP_NOWARN);
+		if (!bvec)
+			return ERR_PTR(-EAGAIN);
+	}
 
 	tmp = bvec;
 	rq_for_each_bvec(bv, rq, rq_iter) {
 		*tmp = bv;
 		tmp++;
 	}
-out:
 	return bvec;
 }
 ALLOW_ERROR_INJECTION(ploop_create_bvec_from_rq, NULL);
@@ -2003,9 +2023,15 @@ static void ploop_prepare_one_embedded_pio(struct ploop *ploop,
 		 * Transform a set of bvec arrays related to bios
 		 * into a single bvec array (which we can iterate).
 		 */
-		bvec = ploop_create_bvec_from_rq(rq);
-		if (!bvec)
+		bvec = ploop_create_bvec_from_rq(rq, pio->use_kvmalloc);
+		if (IS_ERR(bvec)) {
+			if (PTR_ERR(bvec) == -EAGAIN) {
+				pio->use_kvmalloc = true;
+				llist_add((struct llist_node *)(&pio->list), &ploop->pios[PLOOP_LIST_PREPARE]);
+				return;
+			}
 			goto err_nomem;
+		}
 		prq->bvec = bvec;
 skip_bvec:
 		pio->bi_iter.bi_size = blk_rq_bytes(rq);
diff --git a/drivers/md/dm-ploop.h b/drivers/md/dm-ploop.h
index fc12efeb0cd9..53e8d12064bd 100644
--- a/drivers/md/dm-ploop.h
+++ b/drivers/md/dm-ploop.h
@@ -316,6 +316,7 @@ struct pio {
 	unsigned int ref_index:2;
 
 	u8 queue_list_id; /* id in ploop->pios */
+	bool use_kvmalloc;
 
 	struct ploop_index_wb *piwb;
 
-- 
2.43.0