bun.sh/src/allocators/LinuxMemFdAllocator.zig

//! When cloning large amounts of data potentially multiple times, we can
//! leverage copy-on-write memory to avoid actually copying the data. To do that
//! on Linux, we need to use a memfd, which is a Linux-specific feature.
//!
//! The steps are roughly:
//!
//! 1. Create a memfd
//! 2. Write the data to the memfd
//! 3. Map the memfd into memory
//!
//! Then, to clone the data later, we can just call `mmap` again.
//!
//! The big catch is that mmap(), memfd_create(), write() all have overhead. And
//! often we will re-use virtual memory within the process. This does not reuse
//! the virtual memory. So we should only really use this for large blobs of
//! data that we expect to be cloned multiple times. Such as Blob in FormData.

const Self = @This();

const RefCount = bun.ptr.ThreadSafeRefCount(@This(), "ref_count", deinit, .{});

pub const new = bun.TrivialNew(@This());
pub const ref = RefCount.ref;
pub const deref = RefCount.deref;

ref_count: RefCount,
fd: bun.FileDescriptor = .invalid,
size: usize = 0,

var memfd_counter = std.atomic.Value(usize).init(0);

fn deinit(self: *Self) void {
    self.fd.close();
    bun.destroy(self);
}

pub fn allocator(self: *Self) std.mem.Allocator {
    return .{
        .ptr = self,
        .vtable = AllocatorInterface.VTable,
    };
}

pub fn from(allocator_: std.mem.Allocator) ?*Self {
    if (allocator_.vtable == AllocatorInterface.VTable) {
        return @alignCast(@ptrCast(allocator_.ptr));
    }

    return null;
}

const AllocatorInterface = struct {
    fn alloc(_: *anyopaque, _: usize, _: std.mem.Alignment, _: usize) ?[*]u8 {
        // it should perform no allocations or resizes
        return null;
    }

    fn free(
        ptr: *anyopaque,
        buf: []u8,
        _: std.mem.Alignment,
        _: usize,
    ) void {
        var self: *Self = @alignCast(@ptrCast(ptr));
        defer self.deref();
        bun.sys.munmap(@alignCast(@ptrCast(buf))).unwrap() catch |err| {
            bun.Output.debugWarn("Failed to munmap memfd: {}", .{err});
        };
    }

    pub const VTable = &std.mem.Allocator.VTable{
        .alloc = &AllocatorInterface.alloc,
        .resize = &std.mem.Allocator.noResize,
        .remap = &std.mem.Allocator.noRemap,
        .free = &free,
    };
};

pub fn alloc(self: *Self, len: usize, offset: usize, flags: std.posix.MAP) bun.jsc.Maybe(bun.webcore.Blob.Store.Bytes) {
    var size = len;

    // size rounded up to nearest page
    size = std.mem.alignForward(usize, size, std.heap.pageSize());

    var flags_mut = flags;
    flags_mut.TYPE = .SHARED;

    switch (bun.sys.mmap(
        null,
        @min(size, self.size),
        std.posix.PROT.READ | std.posix.PROT.WRITE,
        flags_mut,
        self.fd,
        offset,
    )) {
        .result => |slice| {
            return .{
                .result = bun.webcore.Blob.Store.Bytes{
                    .cap = @truncate(slice.len),
                    .ptr = slice.ptr,
                    .len = @truncate(len),
                    .allocator = self.allocator(),
                },
            };
        },
        .err => |errno| {
            return .{ .err = errno };
        },
    }
}

pub fn shouldUse(bytes: []const u8) bool {
    if (comptime !bun.Environment.isLinux) {
        return false;
    }

    if (bun.jsc.VirtualMachine.is_smol_mode) {
        return bytes.len >= 1024 * 1024 * 1;
    }

    // This is a net 2x - 4x slowdown to new Blob([huge])
    // so we must be careful
    return bytes.len >= 1024 * 1024 * 8;
}

pub fn create(bytes: []const u8) bun.jsc.Maybe(bun.webcore.Blob.Store.Bytes) {
    if (comptime !bun.Environment.isLinux) {
        unreachable;
    }

    var label_buf: [128]u8 = undefined;
    const label = std.fmt.bufPrintZ(&label_buf, "memfd-num-{d}", .{memfd_counter.fetchAdd(1, .monotonic)}) catch "";

    // Using huge pages was slower.
    const fd = switch (bun.sys.memfd_create(label, std.os.linux.MFD.CLOEXEC)) {
        .err => |err| return .{ .err = bun.sys.Error.fromCode(err.getErrno(), .open) },
        .result => |fd| fd,
    };

    if (bytes.len > 0)
        // Hint at the size of the file
        _ = bun.sys.ftruncate(fd, @intCast(bytes.len));

    // Dump all the bytes in there
    var written: isize = 0;

    var remain = bytes;
    while (remain.len > 0) {
        switch (bun.sys.pwrite(fd, remain, written)) {
            .err => |err| {
                if (err.getErrno() == .AGAIN) {
                    continue;
                }

                bun.Output.debugWarn("Failed to write to memfd: {}", .{err});
                fd.close();
                return .{ .err = err };
            },
            .result => |result| {
                if (result == 0) {
                    bun.Output.debugWarn("Failed to write to memfd: EOF", .{});
                    fd.close();
                    return .{ .err = bun.sys.Error.fromCode(.NOMEM, .write) };
                }
                written += @intCast(result);
                remain = remain[result..];
            },
        }
    }

    var linux_memfd_allocator = Self.new(.{
        .fd = fd,
        .ref_count = .init(),
        .size = bytes.len,
    });

    switch (linux_memfd_allocator.alloc(bytes.len, 0, .{ .TYPE = .SHARED })) {
        .result => |res| {
            return .{ .result = res };
        },
        .err => |err| {
            linux_memfd_allocator.deref();
            return .{ .err = err };
        },
    }
}

const bun = @import("bun");
const std = @import("std");