[RFC v2 0/5] md/raid1: introduce a new sync action to repair badblocks

From: Zheng Qixing <[email protected]>

Hi,

This is v2 of the series.

# Mechanism
When rectifying badblocks, we issue a single repair write for
the bad range (copy data from a good mirror to the corresponding
LBA on bad mirror). Once the write completes successfully
(bi_status == 0), the LBA range is cleared from the badblocks
table. If the media is still bad for that LBA, a subsequent
read/write will fail again and the range will be marked bad
again.

Doing a read-back for every repair would only prove that the data
is readable at that moment, and it does not provide a stronger
guarantee against future internal remapping.

# Why use LBS granularity for bad-block repair?
In our RAID1 bad-block repair (rectify) testing on a device
reporting 512B logical blocks and 4KiB physical blocks, we
issue 512B I/O directly to the md device and inject an I/O
fault.

Since the md badblocks table can only track failures in terms
of host-visible LBA ranges, it is updated at 512B sector
granularity (i.e., it records the failing sector) and does not
attempt to infer or expand the entry to a 4KiB physical-block
boundary.

Given that the OS has no visibility into the device's internal
mapping from LBAs to physical media (or the FTL), using
logical block size for recording and repairing bad blocks is
the most appropriate choice from a correctness standpoint.

If the underlying media failure is actually larger than 512B,
this is typically reflected by subsequent failures on adjacent
LBAs, at which point the recorded bad range will naturally
grow to cover the affected area.

# Tests
This feature has been tested on a RAID1 built from two 480GB
system disks. It has also been tested under QEMU with a 4-disk
RAID1 setup, with both memory fault injection and I/O fault
injection enabled.

In addition, we will add a new test (26raid1-rectify-badblocks)
in mdadm/tests to verify whether `rectify` can effectively repair
sectors recorded in bad_blocks.

# TODO
rectify currently only supports bad-block repair for the RAID1
level. We will consider extending it to RAID5/10 in follow-up
work.

Changes in v2:
 - Patch 1: Remove non-essential helpers to reduce indirection.
 - Patch 2: Split out a bugfix that was previously included in patch 1.
 - Patch 3: Rename the /proc/mdstat action from "recovery" to "recover"
   to match the naming used by action_store() and action_show().
 - Patch 4: Add a brief comment for MAX_RAID_DISKS.
 - Patch 5: For rectify, reuse handle_sync_write_finished() to handle
   write request completion, removing duplicate completion handling.

Link of v1:
  https://lore.kernel.org/all/[email protected]/

Zheng Qixing (5):
  md: add helpers for requested sync action
  md: serialize requested sync actions and clear stale request state
  md: rename mdstat action "recovery" to "recover"
  md: introduce MAX_RAID_DISKS macro to replace magic number
  md/raid1: introduce rectify action to repair badblocks

 drivers/md/md.c    | 152 +++++++++++++++++++------
 drivers/md/md.h    |  21 ++++
 drivers/md/raid1.c | 270 ++++++++++++++++++++++++++++++++++++++++++++-
 drivers/md/raid1.h |   1 +
 4 files changed, 409 insertions(+), 35 deletions(-)

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2.39.2