add #![deny(missing_docs)] AND #![deny(warnings)] in main.rs, and add more comments

os/src/syscall/fs.rs

@@ -1,5 +1,8 @@
+//! File and filesystem-related syscalls
+
 const FD_STDOUT: usize = 1;
 
+/// write buf of length `len`  to a file with `fd`
 pub fn sys_write(fd: usize, buf: *const u8, len: usize) -> isize {
     match fd {
         FD_STDOUT => {

os/src/syscall/mod.rs

@@ -1,3 +1,15 @@
+//! Implementation of syscalls
+//!
+//! The single entry point to all system calls, [`syscall()`], is called
+//! whenever userspace wishes to perform a system call using the `ecall`
+//! instruction. In this case, the processor raises an 'Environment call from
+//! U-mode' exception, which is handled as one of the cases in
+//! [`crate::trap::trap_handler`].
+//!
+//! For clarity, each single syscall is implemented as its own function, named
+//! `sys_` then the name of the syscall. You can find functions like this in
+//! submodules, and you should also implement syscalls this way.
+
 const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
 const SYSCALL_YIELD: usize = 124;

os/src/syscall/process.rs

@@ -1,3 +1,4 @@
+//! Process management syscalls
 use crate::task::{exit_current_and_run_next, suspend_current_and_run_next};
 use crate::timer::get_time_ms;