block: Fall back to write when fallocate returns EOPNOTSUPP

Filesystems such as tmpfs do not support fallocate with
FALLOC_FL_ZERO_RANGE or FALLOC_FL_PUNCH_HOLE and return EOPNOTSUPP.
When a raw disk image lives on such a filesystem, virtio write zeroes
and discard requests fail with IOERR.

Use the WriteZeroesAt trait from vmm_sys_util through AlignedFile,
which already bundles fallocate with a positional write fallback.
For punch_hole, catch EOPNOTSUPP and fall back to the same trait.

The io_uring engine previously submitted fallocate directly through
the ring, where the async EOPNOTSUPP completion had no retry path.
Route it through the same sync helpers that the other engines
already use.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/formats/raw/engine_aio.rs

@@ -94,13 +94,8 @@ impl AsyncIo for RawAio {
         // Linux AIO has no IOCB command for fallocate, so perform the
         // operation synchronously and signal completion via the completion
         // list, matching the pattern used by the sync backend (RawSync).
-        punch_hole(
-            self.raw_file.as_raw_fd(),
-            self.is_block_device,
-            offset,
-            length,
-        )
-        .map_err(AsyncIoError::PunchHole)?;
+        punch_hole(&mut self.raw_file, self.is_block_device, offset, length)
+            .map_err(AsyncIoError::PunchHole)?;
         self.data_io
             .inject_completion(AsyncIoCompletion::new(user_data, 0, None));
 
@@ -109,13 +104,8 @@ impl AsyncIo for RawAio {
 
     fn write_zeroes(&mut self, offset: u64, length: u64, user_data: u64) -> AsyncIoResult<()> {
         // Same as punch_hole().
-        write_zeroes(
-            self.raw_file.as_raw_fd(),
-            self.is_block_device,
-            offset,
-            length,
-        )
-        .map_err(AsyncIoError::WriteZeroes)?;
+        write_zeroes(&mut self.raw_file, self.is_block_device, offset, length)
+            .map_err(AsyncIoError::WriteZeroes)?;
         self.data_io
             .inject_completion(AsyncIoCompletion::new(user_data, 0, None));
 

block/src/formats/raw/engine_sync.rs

@@ -120,13 +120,8 @@ impl AsyncIo for RawSync {
     }
 
     fn punch_hole(&mut self, offset: u64, length: u64, user_data: u64) -> AsyncIoResult<()> {
-        punch_hole(
-            self.raw_file.as_raw_fd(),
-            self.is_block_device,
-            offset,
-            length,
-        )
-        .map_err(AsyncIoError::PunchHole)?;
+        punch_hole(&mut self.raw_file, self.is_block_device, offset, length)
+            .map_err(AsyncIoError::PunchHole)?;
         self.completion_list
             .push_back(AsyncIoCompletion::new(user_data, 0, None));
         self.eventfd.write(1).unwrap();
@@ -134,13 +129,8 @@ impl AsyncIo for RawSync {
     }
 
     fn write_zeroes(&mut self, offset: u64, length: u64, user_data: u64) -> AsyncIoResult<()> {
-        write_zeroes(
-            self.raw_file.as_raw_fd(),
-            self.is_block_device,
-            offset,
-            length,
-        )
-        .map_err(AsyncIoError::WriteZeroes)?;
+        write_zeroes(&mut self.raw_file, self.is_block_device, offset, length)
+            .map_err(AsyncIoError::WriteZeroes)?;
         self.completion_list
             .push_back(AsyncIoCompletion::new(user_data, 0, None));
         self.eventfd.write(1).unwrap();

block/src/formats/raw/engine_uring.rs

@@ -6,7 +6,6 @@
 
 use std::os::unix::io::AsRawFd;
 
-use libc::{FALLOC_FL_KEEP_SIZE, FALLOC_FL_PUNCH_HOLE, FALLOC_FL_ZERO_RANGE};
 use vmm_sys_util::eventfd::EventFd;
 
 use super::{operation_is_aligned, run_unaligned_operation};
@@ -14,7 +13,7 @@ use crate::async_io::{
     AsyncIo, AsyncIoCompletion, AsyncIoError, AsyncIoOperation, AsyncIoResult, UringDataIo,
 };
 use crate::error::{BlockError, BlockErrorKind, BlockResult};
-use crate::sparse::{blkdiscard, blkzeroout};
+use crate::sparse::{punch_hole, write_zeroes};
 use crate::{AlignedFile, is_block_device};
 
 pub(crate) struct RawAsync {
@@ -119,44 +118,27 @@ impl AsyncIo for RawAsync {
     }
 
     fn punch_hole(&mut self, offset: u64, length: u64, user_data: u64) -> AsyncIoResult<()> {
-        // Some block devices don't support fallocate(). Use ioctl instead. The assumption is that
-        // this happens rarely and we don't need to introduce unnecessary complexity by submitting
-        // a fallocate request, reaping ENOTSUPP in the completion routine, and reissuing the
-        // request with an ioctl.
-        if self.is_block_device {
-            blkdiscard(self.raw_file.as_raw_fd(), offset, length)
-                .map_err(AsyncIoError::PunchHole)?;
-            // Deliver the completion through the normal io_uring path by
-            // queuing a NOP carrying `user_data`. The registered eventfd will
-            // fire when it completes, just like any other request.
-            return self
-                .data_io
-                .submit_nop(user_data)
-                .map_err(AsyncIoError::PunchHole);
-        }
-
-        let mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
-
+        // Run synchronously rather than submitting a fallocate request through
+        // the ring. This avoids reaping ENOTSUPP in the completion routine and
+        // reissuing the request, and lets the sparse helper handle the ioctl
+        // path for block devices and the write fallback for unsupported
+        // filesystems.
+        punch_hole(&mut self.raw_file, self.is_block_device, offset, length)
+            .map_err(AsyncIoError::PunchHole)?;
+        // Deliver the completion through the normal io_uring path by
+        // queuing a NOP carrying `user_data`. The registered eventfd will
+        // fire when it completes, just like any other request.
         self.data_io
-            .submit_fallocate(self.raw_file.as_raw_fd(), offset, length, mode, user_data)
+            .submit_nop(user_data)
             .map_err(AsyncIoError::PunchHole)
     }
 
     fn write_zeroes(&mut self, offset: u64, length: u64, user_data: u64) -> AsyncIoResult<()> {
         // Same rationale as punch_hole().
-        if self.is_block_device {
-            blkzeroout(self.raw_file.as_raw_fd(), offset, length)
-                .map_err(AsyncIoError::WriteZeroes)?;
-            return self
-                .data_io
-                .submit_nop(user_data)
-                .map_err(AsyncIoError::WriteZeroes);
-        }
-
-        let mode = FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE;
-
+        write_zeroes(&mut self.raw_file, self.is_block_device, offset, length)
+            .map_err(AsyncIoError::WriteZeroes)?;
         self.data_io
-            .submit_fallocate(self.raw_file.as_raw_fd(), offset, length, mode, user_data)
+            .submit_nop(user_data)
             .map_err(AsyncIoError::WriteZeroes)
     }
 }

block/src/io/async_io/uring_data_io.rs

@@ -211,25 +211,6 @@ impl UringDataIo {
         )
     }
 
-    /// Submits a fallocate operation carrying `user_data`.
-    pub fn submit_fallocate(
-        &mut self,
-        fd: RawFd,
-        offset: u64,
-        length: u64,
-        mode: i32,
-        user_data: u64,
-    ) -> io::Result<()> {
-        self.submit_kernel_entry(
-            user_data,
-            &opcode::Fallocate::new(types::Fd(fd), length)
-                .offset(offset)
-                .mode(mode)
-                .build()
-                .user_data(user_data),
-        )
-    }
-
     /// Injects a completion that did not come from a kernel CQE.
     ///
     /// The notifier is signaled so callers can drain it with
@@ -360,13 +341,6 @@ mod tests {
             data_io.submit_nop(7).unwrap_err().kind(),
             io::ErrorKind::AlreadyExists
         );
-        assert_eq!(
-            data_io
-                .submit_fallocate(fd, 0, 512, 0, 7)
-                .unwrap_err()
-                .kind(),
-            io::ErrorKind::AlreadyExists
-        );
 
         let completion = wait_for_completion(&mut data_io);
         assert_eq!(completion.user_data, 7);

block/src/sparse.rs

@@ -19,9 +19,11 @@
 // `__u64[2]` array rather than two separate `*const u64` pointers.
 
 use std::io;
-use std::os::unix::io::RawFd;
+use std::os::unix::io::{AsRawFd, RawFd};
 
-use libc::{FALLOC_FL_KEEP_SIZE, FALLOC_FL_PUNCH_HOLE, FALLOC_FL_ZERO_RANGE};
+use vmm_sys_util::write_zeroes::{PunchHole, WriteZeroesAt};
+
+use crate::AlignedFile;
 
 // `_IO(0x12, 119)` — issue a discard request to a block device.
 pub const BLKDISCARD: libc::c_ulong = 0x1277;
@@ -61,44 +63,45 @@ pub(crate) fn blkzeroout(fd: RawFd, offset: u64, length: u64) -> io::Result<()>
 //
 // On block devices the kernel rejects `fallocate(PUNCH_HOLE)` (notably ZFS
 // zvols), so route through `BLKDISCARD` instead. On regular files use
-// `fallocate(FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE)`.
-pub(crate) fn punch_hole(fd: RawFd, is_blkdev: bool, offset: u64, length: u64) -> io::Result<()> {
+// `PunchHole`, falling back to `WriteZeroesAt` on EOPNOTSUPP (e.g. tmpfs).
+pub(crate) fn punch_hole(
+    file: &mut AlignedFile,
+    is_blkdev: bool,
+    offset: u64,
+    length: u64,
+) -> io::Result<()> {
     if is_blkdev {
-        blkdiscard(fd, offset, length)
-    } else {
-        fallocate(
-            fd,
-            FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
-            offset,
-            length,
-        )
+        return match blkdiscard(file.as_raw_fd(), offset, length) {
+            Ok(()) => Ok(()),
+            Err(e) if e.raw_os_error() == Some(libc::EOPNOTSUPP) => Ok(()),
+            Err(e) => Err(e),
+        };
+    }
+    match file.punch_hole(offset, length) {
+        Ok(()) => Ok(()),
+        Err(e) if e.raw_os_error() == Some(libc::EOPNOTSUPP) => {
+            file.write_all_zeroes_at(offset, length as usize)?;
+            Ok(())
+        }
+        Err(e) => Err(e),
     }
 }
 
 // Zero the byte range `[offset, offset + length)` in `fd`.
 //
 // Uses `BLKZEROOUT` on block devices (see [`punch_hole`] for the rationale)
-// and `fallocate(FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE)` on regular
-// files.
-pub(crate) fn write_zeroes(fd: RawFd, is_blkdev: bool, offset: u64, length: u64) -> io::Result<()> {
+// and `WriteZeroesAt` on regular files, which tries fallocate and falls
+// back to positional writes on EOPNOTSUPP (e.g. tmpfs).
+pub(crate) fn write_zeroes(
+    file: &mut AlignedFile,
+    is_blkdev: bool,
+    offset: u64,
+    length: u64,
+) -> io::Result<()> {
     if is_blkdev {
-        blkzeroout(fd, offset, length)
-    } else {
-        fallocate(
-            fd,
-            FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE,
-            offset,
-            length,
-        )
+        return blkzeroout(file.as_raw_fd(), offset, length);
     }
+    file.write_all_zeroes_at(offset, length as usize)?;
+    Ok(())
 }
 
-fn fallocate(fd: RawFd, mode: libc::c_int, offset: u64, length: u64) -> io::Result<()> {
-    // SAFETY: FFI call with a valid fd; fallocate touches no userspace memory.
-    let ret = unsafe { libc::fallocate(fd, mode, offset as libc::off_t, length as libc::off_t) };
-    if ret == 0 {
-        Ok(())
-    } else {
-        Err(io::Error::last_os_error())
-    }
-}