gbl/libstorage/src/algorithm.rs - platform/bootable/libbootloader - Git at Google

 // Copyright 2024, The Android Open Source Project
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 use crate::{
     as_uninit, check_range, is_aligned, is_buffer_aligned, BlockInfo, BlockIo, SliceMaybeUninit,
 };
 use core::cmp::min;
 use liberror::Result;
 use libutils::aligned_subslice;
 use safemath::SafeNum;

 /// Reads from a range at block boundary to an aligned buffer.
 async fn read_aligned_all(
     io: &mut impl BlockIo,
     offset: u64,
     out: &mut (impl SliceMaybeUninit + ?Sized),
 ) -> Result<()> {
     let blk_offset = check_range(io.info(), offset, out.as_ref())?.try_into()?;
     Ok(io.read_blocks(blk_offset, out).await?)
 }

 /// Read with block-aligned offset and aligned buffer. Size don't need to be block aligned.
 ///   |~~~~~~~~~read~~~~~~~~~|
 ///   |---------|---------|---------|
 async fn read_aligned_offset_and_buffer(
     io: &mut impl BlockIo,
     offset: u64,
     out: &mut (impl SliceMaybeUninit + ?Sized),
     scratch: &mut [u8],
 ) -> Result<()> {
     let block_size = SafeNum::from(io.info().block_size);
     debug_assert!(is_aligned(offset, block_size)?);
     debug_assert!(is_buffer_aligned(out.as_ref(), io.info().alignment)?);

     let aligned_read: usize = SafeNum::from(out.len()).round_down(block_size).try_into()?;

     if aligned_read > 0 {
         read_aligned_all(io, offset, out.get_mut(..aligned_read)?).await?;
     }
     let unaligned = out.get_mut(aligned_read..)?;
     if unaligned.is_empty() {
         return Ok(());
     }
     // Read unalinged part.
     let block_scratch = &mut scratch[..block_size.try_into()?];
     let aligned_offset = SafeNum::from(offset) + aligned_read;
     read_aligned_all(io, aligned_offset.try_into()?, block_scratch).await?;
     unaligned.clone_from_slice(as_uninit(&block_scratch[..unaligned.len()]));
     Ok(())
 }

 /// Read with aligned buffer. Offset and size don't need to be block aligned.
 /// Case 1:
 ///            |~~~~~~read~~~~~~~|
 ///        |------------|------------|
 /// Case 2:
 ///          |~~~read~~~|
 ///        |---------------|--------------|
 async fn read_aligned_buffer(
     io: &mut impl BlockIo,
     offset: u64,
     out: &mut (impl SliceMaybeUninit + ?Sized),
     scratch: &mut [u8],
 ) -> Result<()> {
     debug_assert!(is_buffer_aligned(out.as_ref(), io.info().alignment)?);

     if is_aligned(offset, io.info().block_size)? {
         return read_aligned_offset_and_buffer(io, offset, out, scratch).await;
     }
     let offset = SafeNum::from(offset);
     let aligned_start: u64 =
         min(offset.round_up(io.info().block_size).try_into()?, (offset + out.len()).try_into()?);

     let aligned_relative_offset: usize = (SafeNum::from(aligned_start) - offset).try_into()?;
     if aligned_relative_offset < out.len() {
         if is_buffer_aligned(&out.get(aligned_relative_offset..)?, io.info().alignment)? {
             // If new output address is aligned, read directly.
             read_aligned_offset_and_buffer(
                 io,
                 aligned_start,
                 out.get_mut(aligned_relative_offset..)?,
                 scratch,
             )
             .await?;
         } else {
             // Otherwise read into `out` (assumed aligned) and memmove to the correct
             // position
             let read_len: usize =
                 (SafeNum::from(out.len()) - aligned_relative_offset).try_into()?;
             read_aligned_offset_and_buffer(io, aligned_start, out.get_mut(..read_len)?, scratch)
                 .await?;
             out.as_mut().copy_within(..read_len, aligned_relative_offset);
         }
     }

     // Now read the unaligned part
     let block_scratch = &mut scratch[..SafeNum::from(io.info().block_size).try_into()?];
     let round_down_offset = offset.round_down(io.info().block_size);
     read_aligned_all(io, round_down_offset.try_into()?, block_scratch).await?;
     let offset_relative = offset - round_down_offset;
     let unaligned = out.get_mut(..aligned_relative_offset)?;
     unaligned.clone_from_slice(as_uninit(
         &block_scratch
             [offset_relative.try_into()?..(offset_relative + unaligned.len()).try_into()?],
     ));
     Ok(())
 }

 // Partition a scratch into two aligned parts: [u8; alignment()-1] and [u8; block_size())]
 // for handling block and buffer misalignment respecitvely.
 fn split_scratch<'a>(
     info: BlockInfo,
     scratch: &'a mut [u8],
 ) -> Result<(&'a mut [u8], &'a mut [u8])> {
     let (buffer_alignment, block_alignment) = aligned_subslice(scratch, info.alignment)?
         .split_at_mut((SafeNum::from(info.alignment) - 1).try_into()?);
     let block_alignment = aligned_subslice(block_alignment, info.alignment)?;
     let block_alignment_scratch_size = match info.block_size {
         1 => SafeNum::ZERO,
         v => v.into(),
     };
     Ok((buffer_alignment, &mut block_alignment[..block_alignment_scratch_size.try_into()?]))
 }

 /// Read with no alignment requirement.
 pub async fn read_async(
     io: &mut impl BlockIo,
     offset: u64,
     out: &mut (impl SliceMaybeUninit + ?Sized),
     scratch: &mut [u8],
 ) -> Result<()> {
     let (buffer_alignment_scratch, block_alignment_scratch) = split_scratch(io.info(), scratch)?;

     if is_buffer_aligned(out.as_ref(), io.info().alignment)? {
         return read_aligned_buffer(io, offset, out, block_alignment_scratch).await;
     }

     // Buffer misalignment:
     // Case 1:
     //     |~~~~~~~~~~~~buffer~~~~~~~~~~~~|
     //   |----------------------|---------------------|
     //      io.info().alignment
     //
     // Case 2:
     //    |~~~~~~buffer~~~~~|
     //  |----------------------|---------------------|
     //     io.info().alignment

     let out_addr_value = SafeNum::from(out.as_mut().as_ptr() as usize);
     let unaligned_read: usize =
         min((out_addr_value.round_up(io.info().alignment) - out_addr_value).try_into()?, out.len());

     // Read unaligned part
     let unaligned_out = &mut buffer_alignment_scratch[..unaligned_read];
     read_aligned_buffer(io, offset, unaligned_out, block_alignment_scratch).await?;
     out.get_mut(..unaligned_read)?.clone_from_slice(as_uninit(unaligned_out));

     if unaligned_read == out.len() {
         return Ok(());
     }
     // Read aligned part
     read_aligned_buffer(
         io,
         (SafeNum::from(offset) + unaligned_read).try_into()?,
         out.get_mut(unaligned_read..)?,
         block_alignment_scratch,
     )
     .await
 }

 /// Write bytes from aligned buffer to a block boundary range.
 async fn write_aligned_all(io: &mut impl BlockIo, offset: u64, data: &mut [u8]) -> Result<()> {
     let blk_offset = check_range(io.info(), offset, data)?.try_into()?;
     Ok(io.write_blocks(blk_offset, data).await?)
 }

 /// Write with block-aligned offset and aligned buffer. `data.len()` can be unaligned.
 ///   |~~~~~~~~~size~~~~~~~~~|
 ///   |---------|---------|---------|
 async fn write_aligned_offset_and_buffer(
     io: &mut impl BlockIo,
     offset: u64,
     data: &mut [u8],
     scratch: &mut [u8],
 ) -> Result<()> {
     debug_assert!(is_aligned(offset, io.info().block_size)?);
     debug_assert!(is_buffer_aligned(data, io.info().alignment)?);

     let aligned_write: usize =
         SafeNum::from(data.len()).round_down(io.info().block_size).try_into()?;
     if aligned_write > 0 {
         write_aligned_all(io, offset, &mut data[..aligned_write]).await?;
     }
     let unaligned = &data[aligned_write..];
     if unaligned.len() == 0 {
         return Ok(());
     }

     // Perform read-modify-write for the unaligned part
     let unaligned_start: u64 = (SafeNum::from(offset) + aligned_write).try_into()?;
     let block_scratch = &mut scratch[..SafeNum::from(io.info().block_size).try_into()?];
     read_aligned_all(io, unaligned_start, block_scratch).await?;
     block_scratch[..unaligned.len()].clone_from_slice(unaligned);
     write_aligned_all(io, unaligned_start, block_scratch).await
 }

 // Rotates buffer to the left.
 fn rotate_left(slice: &mut [u8], sz: usize, scratch: &mut [u8]) {
     scratch[..sz].clone_from_slice(&slice[..sz]);
     slice.copy_within(sz.., 0);
     let off = slice.len().checked_sub(sz).unwrap();
     slice[off..].clone_from_slice(&scratch[..sz]);
 }

 // Rotates buffer to the right.
 fn rotate_right(slice: &mut [u8], sz: usize, scratch: &mut [u8]) {
     let off = slice.len().checked_sub(sz).unwrap();
     scratch[..sz].clone_from_slice(&slice[off..]);
     slice.copy_within(..off, sz);
     slice[..sz].clone_from_slice(&scratch[..sz]);
 }

 /// Write with aligned buffer. Offset and size don't need to be block aligned.
 /// Case 1:
 ///            |~~~~~~write~~~~~~~|
 ///        |------------|------------|
 /// Case 2:
 ///          |~~~write~~~|
 ///        |---------------|--------------|
 async fn write_aligned_buffer(
     io: &mut impl BlockIo,
     offset: u64,
     data: &mut [u8],
     scratch: &mut [u8],
 ) -> Result<()> {
     debug_assert!(is_buffer_aligned(data, io.info().alignment)?);

     let offset = SafeNum::from(offset);
     if is_aligned(offset, io.info().block_size)? {
         return write_aligned_offset_and_buffer(io, offset.try_into()?, data, scratch).await;
     }

     let aligned_start: u64 =
         min(offset.round_up(io.info().block_size).try_into()?, (offset + data.len()).try_into()?);
     let aligned_relative_offset: usize = (SafeNum::from(aligned_start) - offset).try_into()?;
     if aligned_relative_offset < data.len() {
         if is_buffer_aligned(&data[aligned_relative_offset..], io.info().alignment)? {
             // If new address is aligned, write directly.
             write_aligned_offset_and_buffer(
                 io,
                 aligned_start,
                 &mut data[aligned_relative_offset..],
                 scratch,
             )
             .await?;
         } else {
             let write_len: usize =
                 (SafeNum::from(data.len()) - aligned_relative_offset).try_into()?;
             // Swap the offset-aligned part to the beginning of the buffer (assumed aligned)
             rotate_left(data, aligned_relative_offset, scratch);
             let res =
                 write_aligned_offset_and_buffer(io, aligned_start, &mut data[..write_len], scratch)
                     .await;
             // Swap the two parts back before checking the result.
             rotate_right(data, aligned_relative_offset, scratch);
             res?;
         }
     }

     // perform read-modify-write for the unaligned part.
     let block_scratch = &mut scratch[..SafeNum::from(io.info().block_size).try_into()?];
     let round_down_offset: u64 = offset.round_down(io.info().block_size).try_into()?;
     read_aligned_all(io, round_down_offset, block_scratch).await?;
     let offset_relative = offset - round_down_offset;
     block_scratch
         [offset_relative.try_into()?..(offset_relative + aligned_relative_offset).try_into()?]
         .clone_from_slice(&data[..aligned_relative_offset]);
     write_aligned_all(io, round_down_offset, block_scratch).await
 }

 /// Writes bytes to the block device.
 /// It does internal optimization that temporarily modifies `data` layout to minimize number of
 /// calls to `io.read_blocks()`/`io.write_blocks()` (down to O(1)).
 pub async fn write_async(
     io: &mut impl BlockIo,
     offset: u64,
     data: &mut [u8],
     scratch: &mut [u8],
 ) -> Result<()> {
     let (buffer_alignment_scratch, block_alignment_scratch) = split_scratch(io.info(), scratch)?;
     if is_buffer_aligned(data, io.info().alignment)? {
         return write_aligned_buffer(io, offset, data, block_alignment_scratch).await;
     }

     // Buffer misalignment:
     // Case 1:
     //     |~~~~~~~~~~~~buffer~~~~~~~~~~~~|
     //   |----------------------|---------------------|
     //      io.alignment()
     //
     // Case 2:
     //    |~~~~~~buffer~~~~~|
     //  |----------------------|---------------------|
     //     io.alignment()

     // Write unaligned part
     let data_addr_value = SafeNum::from(data.as_ptr() as usize);
     let unaligned_write: usize = min(
         (data_addr_value.round_up(io.info().alignment) - data_addr_value).try_into()?,
         data.len(),
     );
     let mut unaligned_data = &mut buffer_alignment_scratch[..unaligned_write];
     unaligned_data.clone_from_slice(&data[..unaligned_write]);
     write_aligned_buffer(io, offset, &mut unaligned_data, block_alignment_scratch).await?;
     if unaligned_write == data.len() {
         return Ok(());
     }

     // Write aligned part
     write_aligned_buffer(
         io,
         (SafeNum::from(offset) + unaligned_write).try_into()?,
         &mut data[unaligned_write..],
         block_alignment_scratch,
     )
     .await
 }
	// Copyright 2024, The Android Open Source Project
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	use crate::{
	as_uninit, check_range, is_aligned, is_buffer_aligned, BlockInfo, BlockIo, SliceMaybeUninit,
	};
	use core::cmp::min;
	use liberror::Result;
	use libutils::aligned_subslice;
	use safemath::SafeNum;

	/// Reads from a range at block boundary to an aligned buffer.
	async fn read_aligned_all(
	io: &mut impl BlockIo,
	offset: u64,
	out: &mut (impl SliceMaybeUninit + ?Sized),
	) -> Result<()> {
	let blk_offset = check_range(io.info(), offset, out.as_ref())?.try_into()?;
	Ok(io.read_blocks(blk_offset, out).await?)
	}

	/// Read with block-aligned offset and aligned buffer. Size don't need to be block aligned.
	/// \|~~~~~~~~~read~~~~~~~~~\|
	/// \|---------\|---------\|---------\|
	async fn read_aligned_offset_and_buffer(
	io: &mut impl BlockIo,
	offset: u64,
	out: &mut (impl SliceMaybeUninit + ?Sized),
	scratch: &mut [u8],
	) -> Result<()> {
	let block_size = SafeNum::from(io.info().block_size);
	debug_assert!(is_aligned(offset, block_size)?);
	debug_assert!(is_buffer_aligned(out.as_ref(), io.info().alignment)?);

	let aligned_read: usize = SafeNum::from(out.len()).round_down(block_size).try_into()?;

	if aligned_read > 0 {
	read_aligned_all(io, offset, out.get_mut(..aligned_read)?).await?;
	}
	let unaligned = out.get_mut(aligned_read..)?;
	if unaligned.is_empty() {
	return Ok(());
	}
	// Read unalinged part.
	let block_scratch = &mut scratch[..block_size.try_into()?];
	let aligned_offset = SafeNum::from(offset) + aligned_read;
	read_aligned_all(io, aligned_offset.try_into()?, block_scratch).await?;
	unaligned.clone_from_slice(as_uninit(&block_scratch[..unaligned.len()]));
	Ok(())
	}

	/// Read with aligned buffer. Offset and size don't need to be block aligned.
	/// Case 1:
	/// \|~~~~~~read~~~~~~~\|
	/// \|------------\|------------\|
	/// Case 2:
	/// \|~~~read~~~\|
	/// \|---------------\|--------------\|
	async fn read_aligned_buffer(
	io: &mut impl BlockIo,
	offset: u64,
	out: &mut (impl SliceMaybeUninit + ?Sized),
	scratch: &mut [u8],
	) -> Result<()> {
	debug_assert!(is_buffer_aligned(out.as_ref(), io.info().alignment)?);

	if is_aligned(offset, io.info().block_size)? {
	return read_aligned_offset_and_buffer(io, offset, out, scratch).await;
	}
	let offset = SafeNum::from(offset);
	let aligned_start: u64 =
	min(offset.round_up(io.info().block_size).try_into()?, (offset + out.len()).try_into()?);

	let aligned_relative_offset: usize = (SafeNum::from(aligned_start) - offset).try_into()?;
	if aligned_relative_offset < out.len() {
	if is_buffer_aligned(&out.get(aligned_relative_offset..)?, io.info().alignment)? {
	// If new output address is aligned, read directly.
	read_aligned_offset_and_buffer(
	io,
	aligned_start,
	out.get_mut(aligned_relative_offset..)?,
	scratch,
	)
	.await?;
	} else {
	// Otherwise read into `out` (assumed aligned) and memmove to the correct
	// position
	let read_len: usize =
	(SafeNum::from(out.len()) - aligned_relative_offset).try_into()?;
	read_aligned_offset_and_buffer(io, aligned_start, out.get_mut(..read_len)?, scratch)
	.await?;
	out.as_mut().copy_within(..read_len, aligned_relative_offset);
	}
	}

	// Now read the unaligned part
	let block_scratch = &mut scratch[..SafeNum::from(io.info().block_size).try_into()?];
	let round_down_offset = offset.round_down(io.info().block_size);
	read_aligned_all(io, round_down_offset.try_into()?, block_scratch).await?;
	let offset_relative = offset - round_down_offset;
	let unaligned = out.get_mut(..aligned_relative_offset)?;
	unaligned.clone_from_slice(as_uninit(
	&block_scratch
	[offset_relative.try_into()?..(offset_relative + unaligned.len()).try_into()?],
	));
	Ok(())
	}

	// Partition a scratch into two aligned parts: [u8; alignment()-1] and [u8; block_size())]
	// for handling block and buffer misalignment respecitvely.
	fn split_scratch<'a>(
	info: BlockInfo,
	scratch: &'a mut [u8],
	) -> Result<(&'a mut [u8], &'a mut [u8])> {
	let (buffer_alignment, block_alignment) = aligned_subslice(scratch, info.alignment)?
	.split_at_mut((SafeNum::from(info.alignment) - 1).try_into()?);
	let block_alignment = aligned_subslice(block_alignment, info.alignment)?;
	let block_alignment_scratch_size = match info.block_size {
	1 => SafeNum::ZERO,
	v => v.into(),
	};
	Ok((buffer_alignment, &mut block_alignment[..block_alignment_scratch_size.try_into()?]))
	}

	/// Read with no alignment requirement.
	pub async fn read_async(
	io: &mut impl BlockIo,
	offset: u64,
	out: &mut (impl SliceMaybeUninit + ?Sized),
	scratch: &mut [u8],
	) -> Result<()> {
	let (buffer_alignment_scratch, block_alignment_scratch) = split_scratch(io.info(), scratch)?;

	if is_buffer_aligned(out.as_ref(), io.info().alignment)? {
	return read_aligned_buffer(io, offset, out, block_alignment_scratch).await;
	}

	// Buffer misalignment:
	// Case 1:
	// \|~~~~~~~~~~~~buffer~~~~~~~~~~~~\|
	// \|----------------------\|---------------------\|
	// io.info().alignment
	//
	// Case 2:
	// \|~~~~~~buffer~~~~~\|
	// \|----------------------\|---------------------\|
	// io.info().alignment

	let out_addr_value = SafeNum::from(out.as_mut().as_ptr() as usize);
	let unaligned_read: usize =
	min((out_addr_value.round_up(io.info().alignment) - out_addr_value).try_into()?, out.len());

	// Read unaligned part
	let unaligned_out = &mut buffer_alignment_scratch[..unaligned_read];
	read_aligned_buffer(io, offset, unaligned_out, block_alignment_scratch).await?;
	out.get_mut(..unaligned_read)?.clone_from_slice(as_uninit(unaligned_out));

	if unaligned_read == out.len() {
	return Ok(());
	}
	// Read aligned part
	read_aligned_buffer(
	io,
	(SafeNum::from(offset) + unaligned_read).try_into()?,
	out.get_mut(unaligned_read..)?,
	block_alignment_scratch,
	)
	.await
	}

	/// Write bytes from aligned buffer to a block boundary range.
	async fn write_aligned_all(io: &mut impl BlockIo, offset: u64, data: &mut [u8]) -> Result<()> {
	let blk_offset = check_range(io.info(), offset, data)?.try_into()?;
	Ok(io.write_blocks(blk_offset, data).await?)
	}

	/// Write with block-aligned offset and aligned buffer. `data.len()` can be unaligned.
	/// \|~~~~~~~~~size~~~~~~~~~\|
	/// \|---------\|---------\|---------\|
	async fn write_aligned_offset_and_buffer(
	io: &mut impl BlockIo,
	offset: u64,
	data: &mut [u8],
	scratch: &mut [u8],
	) -> Result<()> {
	debug_assert!(is_aligned(offset, io.info().block_size)?);
	debug_assert!(is_buffer_aligned(data, io.info().alignment)?);

	let aligned_write: usize =
	SafeNum::from(data.len()).round_down(io.info().block_size).try_into()?;
	if aligned_write > 0 {
	write_aligned_all(io, offset, &mut data[..aligned_write]).await?;
	}
	let unaligned = &data[aligned_write..];
	if unaligned.len() == 0 {
	return Ok(());
	}

	// Perform read-modify-write for the unaligned part
	let unaligned_start: u64 = (SafeNum::from(offset) + aligned_write).try_into()?;
	let block_scratch = &mut scratch[..SafeNum::from(io.info().block_size).try_into()?];
	read_aligned_all(io, unaligned_start, block_scratch).await?;
	block_scratch[..unaligned.len()].clone_from_slice(unaligned);
	write_aligned_all(io, unaligned_start, block_scratch).await
	}

	// Rotates buffer to the left.
	fn rotate_left(slice: &mut [u8], sz: usize, scratch: &mut [u8]) {
	scratch[..sz].clone_from_slice(&slice[..sz]);
	slice.copy_within(sz.., 0);
	let off = slice.len().checked_sub(sz).unwrap();
	slice[off..].clone_from_slice(&scratch[..sz]);
	}

	// Rotates buffer to the right.
	fn rotate_right(slice: &mut [u8], sz: usize, scratch: &mut [u8]) {
	let off = slice.len().checked_sub(sz).unwrap();
	scratch[..sz].clone_from_slice(&slice[off..]);
	slice.copy_within(..off, sz);
	slice[..sz].clone_from_slice(&scratch[..sz]);
	}

	/// Write with aligned buffer. Offset and size don't need to be block aligned.
	/// Case 1:
	/// \|~~~~~~write~~~~~~~\|
	/// \|------------\|------------\|
	/// Case 2:
	/// \|~~~write~~~\|
	/// \|---------------\|--------------\|
	async fn write_aligned_buffer(
	io: &mut impl BlockIo,
	offset: u64,
	data: &mut [u8],
	scratch: &mut [u8],
	) -> Result<()> {
	debug_assert!(is_buffer_aligned(data, io.info().alignment)?);

	let offset = SafeNum::from(offset);
	if is_aligned(offset, io.info().block_size)? {
	return write_aligned_offset_and_buffer(io, offset.try_into()?, data, scratch).await;
	}

	let aligned_start: u64 =
	min(offset.round_up(io.info().block_size).try_into()?, (offset + data.len()).try_into()?);
	let aligned_relative_offset: usize = (SafeNum::from(aligned_start) - offset).try_into()?;
	if aligned_relative_offset < data.len() {
	if is_buffer_aligned(&data[aligned_relative_offset..], io.info().alignment)? {
	// If new address is aligned, write directly.
	write_aligned_offset_and_buffer(
	io,
	aligned_start,
	&mut data[aligned_relative_offset..],
	scratch,
	)
	.await?;
	} else {
	let write_len: usize =
	(SafeNum::from(data.len()) - aligned_relative_offset).try_into()?;
	// Swap the offset-aligned part to the beginning of the buffer (assumed aligned)
	rotate_left(data, aligned_relative_offset, scratch);
	let res =
	write_aligned_offset_and_buffer(io, aligned_start, &mut data[..write_len], scratch)
	.await;
	// Swap the two parts back before checking the result.
	rotate_right(data, aligned_relative_offset, scratch);
	res?;
	}
	}

	// perform read-modify-write for the unaligned part.
	let block_scratch = &mut scratch[..SafeNum::from(io.info().block_size).try_into()?];
	let round_down_offset: u64 = offset.round_down(io.info().block_size).try_into()?;
	read_aligned_all(io, round_down_offset, block_scratch).await?;
	let offset_relative = offset - round_down_offset;
	block_scratch
	[offset_relative.try_into()?..(offset_relative + aligned_relative_offset).try_into()?]
	.clone_from_slice(&data[..aligned_relative_offset]);
	write_aligned_all(io, round_down_offset, block_scratch).await
	}

	/// Writes bytes to the block device.
	/// It does internal optimization that temporarily modifies `data` layout to minimize number of
	/// calls to `io.read_blocks()`/`io.write_blocks()` (down to O(1)).
	pub async fn write_async(
	io: &mut impl BlockIo,
	offset: u64,
	data: &mut [u8],
	scratch: &mut [u8],
	) -> Result<()> {
	let (buffer_alignment_scratch, block_alignment_scratch) = split_scratch(io.info(), scratch)?;
	if is_buffer_aligned(data, io.info().alignment)? {
	return write_aligned_buffer(io, offset, data, block_alignment_scratch).await;
	}

	// Buffer misalignment:
	// Case 1:
	// \|~~~~~~~~~~~~buffer~~~~~~~~~~~~\|
	// \|----------------------\|---------------------\|
	// io.alignment()
	//
	// Case 2:
	// \|~~~~~~buffer~~~~~\|
	// \|----------------------\|---------------------\|
	// io.alignment()

	// Write unaligned part
	let data_addr_value = SafeNum::from(data.as_ptr() as usize);
	let unaligned_write: usize = min(
	(data_addr_value.round_up(io.info().alignment) - data_addr_value).try_into()?,
	data.len(),
	);
	let mut unaligned_data = &mut buffer_alignment_scratch[..unaligned_write];
	unaligned_data.clone_from_slice(&data[..unaligned_write]);
	write_aligned_buffer(io, offset, &mut unaligned_data, block_alignment_scratch).await?;
	if unaligned_write == data.len() {
	return Ok(());
	}

	// Write aligned part
	write_aligned_buffer(
	io,
	(SafeNum::from(offset) + unaligned_write).try_into()?,
	&mut data[unaligned_write..],
	block_alignment_scratch,
	)
	.await
	}