kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 114 insertions, 0 deletions

diff --git a/Documentation/admin-guide/device-mapper/writecache.rst b/Documentation/admin-guide/device-mapper/writecache.rst
new file mode 100644
index 000000000..60c16b7fd
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/writecache.rst
@@ -0,0 +1,114 @@
+=================
+Writecache target
+=================
+
+The writecache target caches writes on persistent memory or on SSD. It
+doesn't cache reads because reads are supposed to be cached in page cache
+in normal RAM.
+
+When the device is constructed, the first sector should be zeroed or the
+first sector should contain valid superblock from previous invocation.
+
+Constructor parameters:
+
+1. type of the cache device - "p" or "s"
+	- p - persistent memory
+	- s - SSD
+2. the underlying device that will be cached
+3. the cache device
+4. block size (4096 is recommended; the maximum block size is the page
+   size)
+5. the number of optional parameters (the parameters with an argument
+   count as two)
+
+	start_sector n		(default: 0)
+		offset from the start of cache device in 512-byte sectors
+	high_watermark n	(default: 50)
+		start writeback when the number of used blocks reach this
+		watermark
+	low_watermark x		(default: 45)
+		stop writeback when the number of used blocks drops below
+		this watermark
+	writeback_jobs n	(default: unlimited)
+		limit the number of blocks that are in flight during
+		writeback. Setting this value reduces writeback
+		throughput, but it may improve latency of read requests
+	autocommit_blocks n	(default: 64 for pmem, 65536 for ssd)
+		when the application writes this amount of blocks without
+		issuing the FLUSH request, the blocks are automatically
+		committed
+	autocommit_time ms	(default: 1000)
+		autocommit time in milliseconds. The data is automatically
+		committed if this time passes and no FLUSH request is
+		received
+	fua			(by default on)
+		applicable only to persistent memory - use the FUA flag
+		when writing data from persistent memory back to the
+		underlying device
+	nofua
+		applicable only to persistent memory - don't use the FUA
+		flag when writing back data and send the FLUSH request
+		afterwards
+
+		- some underlying devices perform better with fua, some
+		  with nofua. The user should test it
+	cleaner
+		when this option is activated (either in the constructor
+		arguments or by a message), the cache will not promote
+		new writes (however, writes to already cached blocks are
+		promoted, to avoid data corruption due to misordered
+		writes) and it will gradually writeback any cached
+		data. The userspace can then monitor the cleaning
+		process with "dmsetup status". When the number of cached
+		blocks drops to zero, userspace can unload the
+		dm-writecache target and replace it with dm-linear or
+		other targets.
+	max_age n
+		specifies the maximum age of a block in milliseconds. If
+		a block is stored in the cache for too long, it will be
+		written to the underlying device and cleaned up.
+	metadata_only
+		only metadata is promoted to the cache. This option
+		improves performance for heavier REQ_META workloads.
+	pause_writeback n	(default: 3000)
+		pause writeback if there was some write I/O redirected to
+		the origin volume in the last n milliseconds
+
+Status:
+
+1. error indicator - 0 if there was no error, otherwise error number
+2. the number of blocks
+3. the number of free blocks
+4. the number of blocks under writeback
+5. the number of read blocks
+6. the number of read blocks that hit the cache
+7. the number of write blocks
+8. the number of write blocks that hit uncommitted block
+9. the number of write blocks that hit committed block
+10. the number of write blocks that bypass the cache
+11. the number of write blocks that are allocated in the cache
+12. the number of write requests that are blocked on the freelist
+13. the number of flush requests
+14. the number of discarded blocks
+
+Messages:
+	flush
+		Flush the cache device. The message returns successfully
+		if the cache device was flushed without an error
+	flush_on_suspend
+		Flush the cache device on next suspend. Use this message
+		when you are going to remove the cache device. The proper
+		sequence for removing the cache device is:
+
+		1. send the "flush_on_suspend" message
+		2. load an inactive table with a linear target that maps
+		   to the underlying device
+		3. suspend the device
+		4. ask for status and verify that there are no errors
+		5. resume the device, so that it will use the linear
+		   target
+		6. the cache device is now inactive and it can be deleted
+	cleaner
+		See above "cleaner" constructor documentation.
+	clear_stats
+		Clear the statistics that are reported on the status line