[Devel] [PATCH v3 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 in case the kmalloc_array allocation fails.
This avoids high-order GFP_ATOMIC allocations from interrupt context
and ensures more reliable memory allocation behavior.

Signed-off-by: Vasileios Almpanis <vasileios.almpanis at virtuozzo.com>
Acked-by: Denis V. Lunev <den at openvz.org>

Feature: dm-ploop: ploop target driver
---
 drivers/md/dm-ploop-map.c | 27 ++++++++++++++++++---------
 1 file changed, 18 insertions(+), 9 deletions(-)

diff --git a/drivers/md/dm-ploop-map.c b/drivers/md/dm-ploop-map.c
index 3fb841f8bcea..8e40497b837c 100644
--- a/drivers/md/dm-ploop-map.c
+++ b/drivers/md/dm-ploop-map.c
@@ -194,7 +194,7 @@ static void ploop_prq_endio(struct pio *pio, void *prq_ptr,
 	struct request *rq = prq->rq;
 
 	if (prq->bvec)
-		kfree(prq->bvec);
+		kvfree(prq->bvec);
 	if (prq->css)
 		css_put(prq->css);
 	/*
@@ -1963,7 +1963,7 @@ 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, gfp_t flags)
 {
 	struct bio_vec bv, *bvec, *tmp;
 	struct req_iterator rq_iter;
@@ -1972,8 +1972,7 @@ static struct bio_vec *ploop_create_bvec_from_rq(struct request *rq)
 	rq_for_each_bvec(bv, rq, rq_iter)
 		nr_bvec++;
 
-	bvec = kmalloc_array(nr_bvec, sizeof(struct bio_vec),
-			     GFP_ATOMIC);
+	bvec = kvmalloc_array(nr_bvec, sizeof(struct bio_vec), flags);
 	if (!bvec)
 		goto out;
 
@@ -1989,7 +1988,8 @@ ALLOW_ERROR_INJECTION(ploop_create_bvec_from_rq, NULL);
 
 static void ploop_prepare_one_embedded_pio(struct ploop *ploop,
 					   struct pio *pio,
-					   struct llist_head *lldeferred_pios)
+					   struct llist_head *lldeferred_pios,
+					   gfp_t flags)
 {
 	struct ploop_rq *prq = pio->endio_cb_data;
 	struct request *rq = prq->rq;
@@ -2003,9 +2003,18 @@ 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, flags);
+		if (!bvec) {
+			/*
+			 * If allocation in atomic context fails defer
+			 * it to blocking context.
+			 */
+			if (!gfpflags_allow_blocking(flags)) {
+				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);
@@ -2044,7 +2053,7 @@ static void ploop_prepare_embedded_pios(struct ploop *ploop,
 		pio = list_entry((struct list_head *)pos, typeof(*pio), list);
 		INIT_LIST_HEAD(&pio->list); /* until type is changed */
 		if (pio->queue_list_id != PLOOP_LIST_FLUSH)
-			ploop_prepare_one_embedded_pio(ploop, pio, deferred_pios);
+			ploop_prepare_one_embedded_pio(ploop, pio, deferred_pios, GFP_NOIO);
 		else
 			llist_add((struct llist_node *)(&pio->list),
 				  &ploop->pios[PLOOP_LIST_FLUSH]);
@@ -2615,7 +2624,7 @@ static void ploop_submit_embedded_pio(struct ploop *ploop, struct pio *pio)
 		goto out;
 	}
 
-	ploop_prepare_one_embedded_pio(ploop, pio, &deferred_pios);
+	ploop_prepare_one_embedded_pio(ploop, pio, &deferred_pios, GFP_ATOMIC | __GFP_NOWARN);
 	/*
 	 * Disable fast path due to rcu lockups fs -> ploop -> fs - fses are not reentrant
 	 * we can however try another fast path skip dispatcher thread and pass directly to
-- 
2.43.0