Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 202 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/dm-pcache.rst b/Documentation/admin-guide/device-mapper/dm-pcache.rst
new file mode 100644
index 000000000..09d327ef4
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/dm-pcache.rst
@@ -0,0 +1,202 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+dm-pcache — Persistent Cache
+=================================
+
+*Author: Dongsheng Yang <dongsheng.yang@linux.dev>*
+
+This document describes *dm-pcache*, a Device-Mapper target that lets a
+byte-addressable *DAX* (persistent-memory, “pmem”) region act as a
+high-performance, crash-persistent cache in front of a slower block
+device.  The code lives in `drivers/md/dm-pcache/`.
+
+Quick feature summary
+=====================
+
+* *Write-back* caching (only mode currently supported).
+* *16 MiB segments* allocated on the pmem device.
+* *Data CRC32* verification (optional, per cache).
+* Crash-safe: every metadata structure is duplicated (`PCACHE_META_INDEX_MAX
+  == 2`) and protected with CRC+sequence numbers.
+* *Multi-tree indexing* (indexing trees sharded by logical address) for high PMem parallelism
+* Pure *DAX path* I/O – no extra BIO round-trips
+* *Log-structured write-back* that preserves backend crash-consistency
+
+
+Constructor
+===========
+
+::
+
+    pcache <cache_dev> <backing_dev> [<number_of_optional_arguments> <cache_mode writeback> <data_crc true|false>]
+
+=========================  ====================================================
+``cache_dev``               Any DAX-capable block device (``/dev/pmem0``…).
+                            All metadata *and* cached blocks are stored here.
+
+``backing_dev``             The slow block device to be cached.
+
+``cache_mode``              Optional, Only ``writeback`` is accepted at the
+                            moment.
+
+``data_crc``                Optional, default to ``false``
+
+                            * ``true``  – store CRC32 for every cached entry
+			      and verify on reads
+                            * ``false`` – skip CRC (faster)
+=========================  ====================================================
+
+Example
+-------
+
+.. code-block:: shell
+
+   dmsetup create pcache_sdb --table \
+     "0 $(blockdev --getsz /dev/sdb) pcache /dev/pmem0 /dev/sdb 4 cache_mode writeback data_crc true"
+
+The first time a pmem device is used, dm-pcache formats it automatically
+(super-block, cache_info, etc.).
+
+
+Status line
+===========
+
+``dmsetup status <device>`` (``STATUSTYPE_INFO``) prints:
+
+::
+
+   <sb_flags> <seg_total> <cache_segs> <segs_used> \
+   <gc_percent> <cache_flags> \
+   <key_head_seg>:<key_head_off> \
+   <dirty_tail_seg>:<dirty_tail_off> \
+   <key_tail_seg>:<key_tail_off>
+
+Field meanings
+--------------
+
+===============================  =============================================
+``sb_flags``                     Super-block flags (e.g. endian marker).
+
+``seg_total``                    Number of physical *pmem* segments.
+
+``cache_segs``                   Number of segments used for cache.
+
+``segs_used``                    Segments currently allocated (bitmap weight).
+
+``gc_percent``                   Current GC high-water mark (0-90).
+
+``cache_flags``                  Bit 0 – DATA_CRC enabled
+                                 Bit 1 – INIT_DONE (cache initialised)
+                                 Bits 2-5 – cache mode (0 == WB).
+
+``key_head``                     Where new key-sets are being written.
+
+``dirty_tail``                   First dirty key-set that still needs
+                                 write-back to the backing device.
+
+``key_tail``                     First key-set that may be reclaimed by GC.
+===============================  =============================================
+
+
+Messages
+========
+
+*Change GC trigger*
+
+::
+
+   dmsetup message <dev> 0 gc_percent <0-90>
+
+
+Theory of operation
+===================
+
+Sub-devices
+-----------
+
+====================  =========================================================
+backing_dev             Any block device (SSD/HDD/loop/LVM, etc.).
+cache_dev               DAX device; must expose direct-access memory.
+====================  =========================================================
+
+Segments and key-sets
+---------------------
+
+* The pmem space is divided into *16 MiB segments*.
+* Each write allocates space from a per-CPU *data_head* inside a segment.
+* A *cache-key* records a logical range on the origin and where it lives
+  inside pmem (segment + offset + generation).
+* 128 keys form a *key-set* (kset); ksets are written sequentially in pmem
+  and are themselves crash-safe (CRC).
+* The pair *(key_tail, dirty_tail)* delimit clean/dirty and live/dead ksets.
+
+Write-back
+----------
+
+Dirty keys are queued into a tree; a background worker copies data
+back to the backing_dev and advances *dirty_tail*.  A FLUSH/FUA bio from the
+upper layers forces an immediate metadata commit.
+
+Garbage collection
+------------------
+
+GC starts when ``segs_used >= seg_total * gc_percent / 100``.  It walks
+from *key_tail*, frees segments whose every key has been invalidated, and
+advances *key_tail*.
+
+CRC verification
+----------------
+
+If ``data_crc is enabled`` dm-pcache computes a CRC32 over every cached data
+range when it is inserted and stores it in the on-media key.  Reads
+validate the CRC before copying to the caller.
+
+
+Failure handling
+================
+
+* *pmem media errors* – all metadata copies are read with
+  ``copy_mc_to_kernel``; an uncorrectable error logs and aborts initialisation.
+* *Cache full* – if no free segment can be found, writes return ``-EBUSY``;
+  dm-pcache retries internally (request deferral).
+* *System crash* – on attach, the driver replays ksets from *key_tail* to
+  rebuild the in-core trees; every segment’s generation guards against
+  use-after-free keys.
+
+
+Limitations & TODO
+==================
+
+* Only *write-back* mode; other modes planned.
+* Only FIFO cache invalidate; other (LRU, ARC...) planned.
+* Table reload is not supported currently.
+* Discard planned.
+
+
+Example workflow
+================
+
+.. code-block:: shell
+
+   # 1.  Create devices
+   dmsetup create pcache_sdb --table \
+     "0 $(blockdev --getsz /dev/sdb) pcache /dev/pmem0 /dev/sdb 4 cache_mode writeback data_crc true"
+
+   # 2.  Put a filesystem on top
+   mkfs.ext4 /dev/mapper/pcache_sdb
+   mount /dev/mapper/pcache_sdb /mnt
+
+   # 3.  Tune GC threshold to 80 %
+   dmsetup message pcache_sdb 0 gc_percent 80
+
+   # 4.  Observe status
+   watch -n1 'dmsetup status pcache_sdb'
+
+   # 5.  Shutdown
+   umount /mnt
+   dmsetup remove pcache_sdb
+
+
+``dm-pcache`` is under active development; feedback, bug reports and patches
+are very welcome!