[PATCH for-next v2 18/25] io_uring: limit number hash buckets

[Pavel Begunkov <asml.silence@gmail.com>]

Don't allocate to many hash/cancellation buckets, there might be too
many, clamp it to 8 bits, or 256 * 64B = 16KB. We don't usually have too
many requests, and 256 buckets should be enough, especially since we
do hash search only in the cancellation path.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/io_uring.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 4d94757e6e28..2a7a5db12a0e 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -710,12 +710,12 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 
 	/*
 	 * Use 5 bits less than the max cq entries, that should give us around
-	 * 32 entries per hash list if totally full and uniformly spread.
+	 * 32 entries per hash list if totally full and uniformly spread, but
+	 * don't keep too many buckets to not overconsume memory.
 	 */
-	hash_bits = ilog2(p->cq_entries);
-	hash_bits -= 5;
-	if (hash_bits <= 0)
-		hash_bits = 1;
+	hash_bits = ilog2(p->cq_entries) - 5;
+	hash_bits = clamp(hash_bits, 1, 8);
+
 	ctx->cancel_hash_bits = hash_bits;
 	ctx->cancel_hash =
 		kmalloc((1U << hash_bits) * sizeof(struct io_hash_bucket),
-- 
2.36.1