scsi: target: tcmu: Prevent memory reclaim recursion

Prevent recursion into the IO path under low memory conditions by using
GFP_NOIO in place of GFP_KERNEL when allocating a new command with
tcmu_alloc_cmd() and user ring space with tcmu_get_empty_block().

Link: https://lore.kernel.org/r/20191108082901.417950-1-damien.lemoal@wdc.comReported-by: NMasato Suzuki <masato.suzuki@wdc.com>
Signed-off-by: NDamien Le Moal <damien.lemoal@wdc.com>
Acked-by: NMike Christie <mchristi@redhat.com>
Reviewed-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>

drivers/target/target_core_user.c

@@ -499,7 +499,7 @@ static inline bool tcmu_get_empty_block(struct tcmu_dev *udev,
 			schedule_delayed_work(&tcmu_unmap_work, 0);
 
 		/* try to get new page from the mm */
-		page = alloc_page(GFP_KERNEL);
+		page = alloc_page(GFP_NOIO);
 		if (!page)
 			goto err_alloc;
 
@@ -573,7 +573,7 @@ static struct tcmu_cmd *tcmu_alloc_cmd(struct se_cmd *se_cmd)
 	struct tcmu_dev *udev = TCMU_DEV(se_dev);
 	struct tcmu_cmd *tcmu_cmd;
 
-	tcmu_cmd = kmem_cache_zalloc(tcmu_cmd_cache, GFP_KERNEL);
+	tcmu_cmd = kmem_cache_zalloc(tcmu_cmd_cache, GFP_NOIO);
 	if (!tcmu_cmd)
 		return NULL;
 
@@ -584,7 +584,7 @@ static struct tcmu_cmd *tcmu_alloc_cmd(struct se_cmd *se_cmd)
 	tcmu_cmd_reset_dbi_cur(tcmu_cmd);
 	tcmu_cmd->dbi_cnt = tcmu_cmd_get_block_cnt(tcmu_cmd);
 	tcmu_cmd->dbi = kcalloc(tcmu_cmd->dbi_cnt, sizeof(uint32_t),
-				GFP_KERNEL);
+				GFP_NOIO);
 	if (!tcmu_cmd->dbi) {
 		kmem_cache_free(tcmu_cmd_cache, tcmu_cmd);
 		return NULL;