block: warn if !__GFP_DIRECT_RECLAIM in bio_crypt_set_ctx()

bio_crypt_set_ctx() assumes its gfp_mask argument always includes
__GFP_DIRECT_RECLAIM, so that the mempool_alloc() will always succeed.

For now this assumption is still fine, since no callers violate it.
Making bio_crypt_set_ctx() able to fail would add unneeded complexity.

However, if a caller didn't use __GFP_DIRECT_RECLAIM, it would be very
hard to notice the bug.  Make it easier by adding a WARN_ON_ONCE().
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Satya Tangirala <satyat@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Satya Tangirala <satyat@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-crypto.c

@@ -81,7 +81,15 @@ subsys_initcall(bio_crypt_ctx_init);
 void bio_crypt_set_ctx(struct bio *bio, const struct blk_crypto_key *key,
 		       const u64 dun[BLK_CRYPTO_DUN_ARRAY_SIZE], gfp_t gfp_mask)
 {
-	struct bio_crypt_ctx *bc = mempool_alloc(bio_crypt_ctx_pool, gfp_mask);
+	struct bio_crypt_ctx *bc;
+
+	/*
+	 * The caller must use a gfp_mask that contains __GFP_DIRECT_RECLAIM so
+	 * that the mempool_alloc() can't fail.
+	 */
+	WARN_ON_ONCE(!(gfp_mask & __GFP_DIRECT_RECLAIM));
+
+	bc = mempool_alloc(bio_crypt_ctx_pool, gfp_mask);
 
 	bc->bc_key = key;
 	memcpy(bc->bc_dun, dun, sizeof(bc->bc_dun));