io_uring: use GFP_NOWAIT for overflow CQEs on legacy rings

Allocate the overflowing CQE with GFP_NOWAIT instead of GFP_ATOMIC. This
changes causes allocations to fail earlier in out-of-memory situations,
rather than being deferred. Using GFP_ATOMIC allows a process to exceed
memory limits.

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220794
Signed-off-by: Alexandre Negrel <alexandre@negrel.dev>
Link: https://lore.kernel.org/io-uring/20251229201933.515797-1-alexandre@negrel.dev/
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: negrel

io_uring/io_uring.c

@@ -864,7 +864,7 @@ static __cold bool io_cqe_overflow_locked(struct io_ring_ctx *ctx,
 {
 	struct io_overflow_cqe *ocqe;
 
-	ocqe = io_alloc_ocqe(ctx, cqe, big_cqe, GFP_ATOMIC);
+	ocqe = io_alloc_ocqe(ctx, cqe, big_cqe, GFP_NOWAIT);
 	return io_cqring_add_overflow(ctx, ocqe);
 }