From: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com> mainline inclusion from mainline-v6.15-rc4 commit 0bf1f51e34c4e260095fc7306c564c7fe01503e3 category: feature bugzilla: https://atomgit.com/src-openeuler/kernel/issues/8862 Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i... -------------------------------- Add an initial documentation around atomic writes support in ext4. Acked-by: Darrick J. Wong <djwong@kernel.org> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com> Link: https://patch.msgid.link/d3893b9f5ad70317abae72046e81e4c180af91bf.1747337952... Signed-off-by: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Long Li <leo.lilong@huawei.com> --- .../filesystems/ext4/atomic_writes.rst | 225 ++++++++++++++++++ Documentation/filesystems/ext4/overview.rst | 1 + 2 files changed, 226 insertions(+) create mode 100644 Documentation/filesystems/ext4/atomic_writes.rst diff --git a/Documentation/filesystems/ext4/atomic_writes.rst b/Documentation/filesystems/ext4/atomic_writes.rst new file mode 100644 index 000000000000..f65767df3620 --- /dev/null +++ b/Documentation/filesystems/ext4/atomic_writes.rst @@ -0,0 +1,225 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. _atomic_writes: + +Atomic Block Writes +------------------------- + +Introduction +~~~~~~~~~~~~ + +Atomic (untorn) block writes ensure that either the entire write is committed +to disk or none of it is. This prevents "torn writes" during power loss or +system crashes. The ext4 filesystem supports atomic writes (only with Direct +I/O) on regular files with extents, provided the underlying storage device +supports hardware atomic writes. This is supported in the following two ways: + +1. **Single-fsblock Atomic Writes**: + EXT4's supports atomic write operations with a single filesystem block since + v6.13. In this the atomic write unit minimum and maximum sizes are both set + to filesystem blocksize. + e.g. doing atomic write of 16KB with 16KB filesystem blocksize on 64KB + pagesize system is possible. + +2. **Multi-fsblock Atomic Writes with Bigalloc**: + EXT4 now also supports atomic writes spanning multiple filesystem blocks + using a feature known as bigalloc. The atomic write unit's minimum and + maximum sizes are determined by the filesystem block size and cluster size, + based on the underlying device’s supported atomic write unit limits. + +Requirements +~~~~~~~~~~~~ + +Basic requirements for atomic writes in ext4: + + 1. The extents feature must be enabled (default for ext4) + 2. The underlying block device must support atomic writes + 3. For single-fsblock atomic writes: + + 1. A filesystem with appropriate block size (up to the page size) + 4. For multi-fsblock atomic writes: + + 1. The bigalloc feature must be enabled + 2. The cluster size must be appropriately configured + +NOTE: EXT4 does not support software or COW based atomic write, which means +atomic writes on ext4 are only supported if underlying storage device supports +it. + +Multi-fsblock Implementation Details +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The bigalloc feature changes ext4 to allocate in units of multiple filesystem +blocks, also known as clusters. With bigalloc each bit within block bitmap +represents cluster (power of 2 number of blocks) rather than individual +filesystem blocks. +EXT4 supports multi-fsblock atomic writes with bigalloc, subject to the +following constraints. The minimum atomic write size is the larger of the fs +block size and the minimum hardware atomic write unit; and the maximum atomic +write size is smaller of the bigalloc cluster size and the maximum hardware +atomic write unit. Bigalloc ensures that all allocations are aligned to the +cluster size, which satisfies the LBA alignment requirements of the hardware +device if the start of the partition/logical volume is itself aligned correctly. + +Here is the block allocation strategy in bigalloc for atomic writes: + + * For regions with fully mapped extents, no additional work is needed + * For append writes, a new mapped extent is allocated + * For regions that are entirely holes, unwritten extent is created + * For large unwritten extents, the extent gets split into two unwritten + extents of appropriate requested size + * For mixed mapping regions (combinations of holes, unwritten extents, or + mapped extents), ext4_map_blocks() is called in a loop with + EXT4_GET_BLOCKS_ZERO flag to convert the region into a single contiguous + mapped extent by writing zeroes to it and converting any unwritten extents to + written, if found within the range. + +Note: Writing on a single contiguous underlying extent, whether mapped or +unwritten, is not inherently problematic. However, writing to a mixed mapping +region (i.e. one containing a combination of mapped and unwritten extents) +must be avoided when performing atomic writes. + +The reason is that, atomic writes when issued via pwritev2() with the RWF_ATOMIC +flag, requires that either all data is written or none at all. In the event of +a system crash or unexpected power loss during the write operation, the affected +region (when later read) must reflect either the complete old data or the +complete new data, but never a mix of both. + +To enforce this guarantee, we ensure that the write target is backed by +a single, contiguous extent before any data is written. This is critical because +ext4 defers the conversion of unwritten extents to written extents until the I/O +completion path (typically in ->end_io()). If a write is allowed to proceed over +a mixed mapping region (with mapped and unwritten extents) and a failure occurs +mid-write, the system could observe partially updated regions after reboot, i.e. +new data over mapped areas, and stale (old) data over unwritten extents that +were never marked written. This violates the atomicity and/or torn write +prevention guarantee. + +To prevent such torn writes, ext4 proactively allocates a single contiguous +extent for the entire requested region in ``ext4_iomap_alloc`` via +``ext4_map_blocks_atomic()``. EXT4 also force commits the current journalling +transaction in case if allocation is done over mixed mapping. This ensures any +pending metadata updates (like unwritten to written extents conversion) in this +range are in consistent state with the file data blocks, before performing the +actual write I/O. If the commit fails, the whole I/O must be aborted to prevent +from any possible torn writes. +Only after this step, the actual data write operation is performed by the iomap. + +Handling Split Extents Across Leaf Blocks +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +There can be a special edge case where we have logically and physically +contiguous extents stored in separate leaf nodes of the on-disk extent tree. +This occurs because on-disk extent tree merges only happens within the leaf +blocks except for a case where we have 2-level tree which can get merged and +collapsed entirely into the inode. +If such a layout exists and, in the worst case, the extent status cache entries +are reclaimed due to memory pressure, ``ext4_map_blocks()`` may never return +a single contiguous extent for these split leaf extents. + +To address this edge case, a new get block flag +``EXT4_GET_BLOCKS_QUERY_LEAF_BLOCKS flag`` is added to enhance the +``ext4_map_query_blocks()`` lookup behavior. + +This new get block flag allows ``ext4_map_blocks()`` to first check if there is +an entry in the extent status cache for the full range. +If not present, it consults the on-disk extent tree using +``ext4_map_query_blocks()``. +If the located extent is at the end of a leaf node, it probes the next logical +block (lblk) to detect a contiguous extent in the adjacent leaf. + +For now only one additional leaf block is queried to maintain efficiency, as +atomic writes are typically constrained to small sizes +(e.g. [blocksize, clustersize]). + + +Handling Journal transactions +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +To support multi-fsblock atomic writes, we ensure enough journal credits are +reserved during: + + 1. Block allocation time in ``ext4_iomap_alloc()``. We first query if there + could be a mixed mapping for the underlying requested range. If yes, then we + reserve credits of up to ``m_len``, assuming every alternate block can be + an unwritten extent followed by a hole. + + 2. During ``->end_io()`` call, we make sure a single transaction is started for + doing unwritten-to-written conversion. The loop for conversion is mainly + only required to handle a split extent across leaf blocks. + +How to +------ + +Creating Filesystems with Atomic Write Support +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +First check the atomic write units supported by block device. +See :ref:`atomic_write_bdev_support` for more details. + +For single-fsblock atomic writes with a larger block size +(on systems with block size < page size): + +.. code-block:: bash + + # Create an ext4 filesystem with a 16KB block size + # (requires page size >= 16KB) + mkfs.ext4 -b 16384 /dev/device + +For multi-fsblock atomic writes with bigalloc: + +.. code-block:: bash + + # Create an ext4 filesystem with bigalloc and 64KB cluster size + mkfs.ext4 -F -O bigalloc -b 4096 -C 65536 /dev/device + +Where ``-b`` specifies the block size, ``-C`` specifies the cluster size in bytes, +and ``-O bigalloc`` enables the bigalloc feature. + +Application Interface +~~~~~~~~~~~~~~~~~~~~~ + +Applications can use the ``pwritev2()`` system call with the ``RWF_ATOMIC`` flag +to perform atomic writes: + +.. code-block:: c + + pwritev2(fd, iov, iovcnt, offset, RWF_ATOMIC); + +The write must be aligned to the filesystem's block size and not exceed the +filesystem's maximum atomic write unit size. +See ``generic_atomic_write_valid()`` for more details. + +``statx()`` system call with ``STATX_WRITE_ATOMIC`` flag can provides following +details: + + * ``stx_atomic_write_unit_min``: Minimum size of an atomic write request. + * ``stx_atomic_write_unit_max``: Maximum size of an atomic write request. + * ``stx_atomic_write_segments_max``: Upper limit for segments. The number of + separate memory buffers that can be gathered into a write operation + (e.g., the iovcnt parameter for IOV_ITER). Currently, this is always set to one. + +The STATX_ATTR_WRITE_ATOMIC flag in ``statx->attributes`` is set if atomic +writes are supported. + +.. _atomic_write_bdev_support: + +Hardware Support +---------------- + +The underlying storage device must support atomic write operations. +Modern NVMe and SCSI devices often provide this capability. +The Linux kernel exposes this information through sysfs: + +* ``/sys/block/<device>/queue/atomic_write_unit_min`` - Minimum atomic write size +* ``/sys/block/<device>/queue/atomic_write_unit_max`` - Maximum atomic write size + +Nonzero values for these attributes indicate that the device supports +atomic writes. + +See Also +-------- + +* :doc:`bigalloc` - Documentation on the bigalloc feature +* :doc:`allocators` - Documentation on block allocation in ext4 +* Support for atomic block writes in 6.13: + https://lwn.net/Articles/1009298/ diff --git a/Documentation/filesystems/ext4/overview.rst b/Documentation/filesystems/ext4/overview.rst index 0fad6eda6e15..9d4054c17ecb 100644 --- a/Documentation/filesystems/ext4/overview.rst +++ b/Documentation/filesystems/ext4/overview.rst @@ -25,3 +25,4 @@ order. .. include:: inlinedata.rst .. include:: eainode.rst .. include:: verity.rst +.. include:: atomic_writes.rst -- 2.39.2