io-uring: Fix check and reduce MAX_READ_SIZE to 64 blocks

    Reading 64 MB chunks at a time and keeping the kernel busy surpases
    std::fs::read time with unoptimized io_uring one being 1.12% fast

Author: Daksh14

tokio/src/fs/read_uring.rs

@@ -13,7 +13,8 @@ const PROBE_SIZE_U32: u32 = PROBE_SIZE as u32;
 
 // Max bytes we can read using io uring submission at a time
 // SAFETY: cannot be higher than u32::MAX for safe cast
-const MAX_READ_SIZE: usize = u32::MAX as usize;
+// Set to read max 64 blocks at time
+const MAX_READ_SIZE: usize = 64 * 1024 * 1024;
 
 pub(crate) async fn read_uring(path: &Path) -> io::Result<Vec<u8>> {
     let file = OpenOptions::new().read(true).open(path).await?;

tokio/src/io/uring/read.rs

@@ -48,7 +48,7 @@ impl Op<Read> {
     // the caller in terms of size_read can be unsound.
     pub(crate) fn read(fd: OwnedFd, mut buf: Vec<u8>, len: u32, offset: u64) -> Self {
         // don't overwrite on already written part
-        assert!(buf.spare_capacity_mut().len() <= len as usize);
+        assert!(buf.spare_capacity_mut().len() >= len as usize);
         let buf_mut_ptr = buf.spare_capacity_mut().as_mut_ptr().cast();
 
         let read_op = opcode::Read::new(types::Fd(fd.as_raw_fd()), buf_mut_ptr, len)