//! Opting the process out of transparent huge pages.
//!
//! With THP set to `always`, the kernel backs any 2 MiB-aligned anonymous
//! region with a huge page on first touch. Each worker thread's stack mapping
//! qualifies, so a thread that touches a few KiB of stack can hold 2 MiB
//! resident, and `std.Io.Threaded` never retires a worker: one burst on such a
//! host left about 40 of 200 workers at 2 MiB each. nxdns's memory is sized
//! for small pages, so the whole process opts out instead of chasing each
//! mapping.

const std = @import("std");
const linux = std.os.linux;

pub const DisableError = error{
    /// A kernel older than 3.15, which has no such switch.
    Unsupported,
    Unexpected,
};

/// Sets `PR_SET_THP_DISABLE`, which covers every mapping of the process,
/// existing and future, and is inherited across `fork`.
pub fn disable() DisableError!void {
    const rc = linux.prctl(@intFromEnum(linux.PR.SET_THP_DISABLE), 1, 0, 0, 0);
    switch (linux.errno(rc)) {
        .SUCCESS => {},
        .INVAL => return error.Unsupported,
        else => return error.Unexpected,
    }
}

test "disable takes effect for the process" {
    disable() catch |err| switch (err) {
        // qemu-user, which runs the aarch64 suite, rejects the option the way
        // a pre-3.15 kernel does.
        error.Unsupported => return error.SkipZigTest,
        error.Unexpected => return err,
    };
    const rc = linux.prctl(@intFromEnum(linux.PR.GET_THP_DISABLE), 0, 0, 0, 0);
    try std.testing.expectEqual(linux.E.SUCCESS, linux.errno(rc));
    try std.testing.expectEqual(@as(usize, 1), rc);
}