Method query_file_alignment

block/src/lib.rs:768–809

Query the O_DIRECT alignment requirement for a regular file. Uses `statx(STATX_DIOALIGN)` (Linux >= 6.1) to obtain the exact memory and offset alignment the kernel requires for direct I/O on this specific file. Unlike `fstatvfs().f_bsize`, which only returns the filesystem's preferred I/O block size, `STATX_DIOALIGN` reports the true per-file DIO constraints accounting for the filesystem, underly

(f: &File)

    /// the true per-file DIO constraints accounting for the filesystem,
    /// underlying block device, and any stacking (loop, dm, etc.).
    fn query_file_alignment(f: &File) -> u64 {
        // The libc crate does not expose statx / STATX_DIOALIGN on all
        // targets (e.g. musl), so define the constant and a minimal repr(C)
        // struct locally and invoke the syscall directly.
        const STATX_DIOALIGN: u32 = 0x2000;

        // Minimal statx layout, only the needed fields,
        // everything else is padding.
        #[repr(C)]
        struct Statx {
            stx_mask: u32,
            _pad: [u8; 148],
            stx_dio_mem_align: u32,
            stx_dio_offset_align: u32,
            _pad2: [u8; 96],
        }

        let mut stx = mem::MaybeUninit::<Statx>::zeroed();
        // SAFETY: FFI syscall with valid fd and correctly sized buffer.
        let ret = unsafe {
            libc::syscall(
                libc::SYS_statx,
                f.as_raw_fd(),
                c"".as_ptr(),
                libc::AT_EMPTY_PATH,
                STATX_DIOALIGN,
                stx.as_mut_ptr(),
            )
        };
        if ret == 0 {
            // SAFETY: statx succeeded, the struct is fully initialized.
            let stx = unsafe { stx.assume_init() };
            if stx.stx_mask & STATX_DIOALIGN != 0 && stx.stx_dio_mem_align > 0 {
                let align = cmp::max(stx.stx_dio_mem_align, stx.stx_dio_offset_align) as u64;
                debug!("statx(STATX_DIOALIGN) returned alignment {align}");
                return align;
            }
        }

        debug!("O_DIRECT alignment query failed, falling back to default {SECTOR_SIZE}");
        SECTOR_SIZE
    }

    pub fn probe(f: &File) -> std::io::Result<Self> {
        if !Self::is_block_device(f)? {