[Devel] [PATCH RHEL9 COMMIT] FD: dm-ploop: standby mode

[Konstantin Khorenko]

The commit is pushed to "branch-rh9-5.14.0-70.22.1.vz9.17.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh9-5.14.0-70.22.1.vz9.17.9
------>
commit 9f4f187f43399c69c329bd784a2b276c28418372
Author: Konstantin Khorenko <khorenko at virtuozzo.com>
Date:   Mon Nov 7 16:55:05 2022 +0300

    FD: dm-ploop: standby mode
    
    https://jira.sw.ru/browse/PSBM-143049
    
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
    
    Feature: dm-ploop: standby mode
---
 .../FeatureDescriptions/dm-ploop-standby_mode.rst  | 80 ++++++++++++++++++++++
 1 file changed, 80 insertions(+)

diff --git a/Documentation/Virtuozzo/FeatureDescriptions/dm-ploop-standby_mode.rst b/Documentation/Virtuozzo/FeatureDescriptions/dm-ploop-standby_mode.rst
new file mode 100644
index 000000000000..c274756ec0e3
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+++ b/Documentation/Virtuozzo/FeatureDescriptions/dm-ploop-standby_mode.rst
@@ -0,0 +1,80 @@
+======================
+dm-ploop: standby mode
+======================
+
+Applicable configuration:
+=========================
+
+Currently the only example of a configuration which supports ploop
+standby mode is the following:
+ - vStorage is mounted on the Host
+ - ploop device which image is stored on the vStorage
+ - vStorage provides an iSCSI target which is built on that ploop
+   device, the target is handled by the SCST kernel module
+
+Background:
+===========
+
+Under certain circumstances vStorage can return -EBUSY/-ENOTCONN/-EIO on
+a request handling.
+
+Once this happens the ploop is switched to "standby mode" in which it
+does not process userspace requests anymore, and vStorage userspace may
+recover the ploop device by replacing the top delta, and after that the
+ploop device may be functional again.
+
+Implementation details:
+=======================
+
+Switching a ploop to the standby mode is implented via setting the
+QUEUE_FLAG_STANDBY bit to the ploops request queue flags.
+
+ploop standby mode should work only on devices that handle it.
+To achieve this we have introduced a static key to enable the flag
+handling only when key is enabled - checks are at the start and at the
+end of every request and we want to avoid performance impact from these
+checks.
+
+How these static key/standby queue flag are going to be used:
+ - The global "ploop_standby_check" static key is enabled in
+   the SCST module init
+ - SCST sets the QUEUE_STANDBY_EN bit when a ploop device is
+   added to SCST, and clears it when it is deleted from SCST
+
+On systems where we are in mixed mode, meaning we have both devices that
+support the standby flag and devices that do not, a QUEUE_FLAG_STANDBY_EN
+is introduced to indicate the standby support from the device.
+
+Setting the QUEUE_FLAG_STANDBY_EN means the device promises to clear
+QUEUE_FLAG_STANDBY flag when it recovers, so ploop can continue
+processing requests.
+
+To protect from errors we use the static key and the standby_en bit
+as two fuses - if one is off we do not touch anything on the queue.
+If we detect inconsistency at key or bits usage - we just warn
+so it can be fixed.
+
+The state of the flags is exported via /sys/block/*/queue/standby, or
+at /sys/devices/virtual/block/*/queue/standby depending on the device.
+ * not supported - no enabled on the queue
+ * on  - queue is in standby mode, not processing requests
+ * off - queue is processing requests
+
+Differences in vz7/vz9 implementation:
+--------------------------------------
+
+When a ploop is switched to standby mode, its request queue flag
+QUEUE_FLAG_STANDBY is set.
+
+ * Once the bit is detected by SCST, the userspace will initiate
+   recovery by replacing the top delta file without destroying the
+   device, which is why the bit is cleared in ploop_replace_delta()
+   in vz7.
+
+ * In vz9, replace delta is achieved by table reload, which reallocates
+   a new ploop instance, but keeps underlying mapped_device unchanged,
+   thus request_queue unchanged, so we have to clear the bit in
+   ploop_ctr().
+
+https://jira.sw.ru/browse/PSBM-143049
+