[PATCH v4] alloc_tag: expose boot-time compression configuration

Currently, userspace has limited visibility into the exact active
runtime state of memory allocation profiling and its page extension
compression ('sysctl.vm.mem_profiling={0|1|never}[,compressed]').

While reading the sysctl provides basic on/off status, it is currently
impossible for userspace to natively determine whether page-tag
compression was successfully enabled without scraping dmesg boot logs.

Resolve this ambiguity by exposing the active compression state by
adding a new read-only sysctl `vm.mem_profiling_compressed` to output the
state.

Signed-off-by: Abhishek Bapat <[email protected]>
---
v4 change:
- Documentation and commit message refactor.

v3 change:
- Added documentation about the behaviour details of the new sysctl.

v2 change:
- Moved from displaying the state in /proc/allocinfo to a new read-only
sysctl.

 Documentation/mm/allocation-profiling.rst | 11 +++++++++++
 mm/alloc_tag.c                            |  6 ++++++
 2 files changed, 17 insertions(+)

diff --git a/Documentation/mm/allocation-profiling.rst b/Documentation/mm/allocation-profiling.rst
index c3a28467955f..f28358af927a 100644
--- a/Documentation/mm/allocation-profiling.rst
+++ b/Documentation/mm/allocation-profiling.rst
@@ -43,6 +43,17 @@ sysctl:
   warnings produced by allocations made while profiling is disabled and freed
   when it's enabled.
 
+  /proc/sys/vm/mem_profiling_compressed
+
+  1: Page alloc tag compression is enabled.
+
+  0: Page alloc tag compression is disabled.
+
+  This reflects a static boot-time configuration of how page allocation tags are
+  stored (in page flags when compression is enabled and in page_ext when disabled).
+  Toggling `mem_profiling` at runtime does not change the state of
+  `mem_profiling_compressed`.
+
 Runtime info:
   /proc/allocinfo
 
diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c
index 52aece27b00e..877068241f06 100644
--- a/mm/alloc_tag.c
+++ b/mm/alloc_tag.c
@@ -1303,6 +1303,12 @@ static const struct ctl_table memory_allocation_profiling_sysctls[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_mem_profiling_handler,
 	},
+	{
+		.procname	= "mem_profiling_compressed",
+		.data		= &mem_profiling_compressed,
+		.mode		= 0444,
+		.proc_handler	= proc_do_static_key,
+	},
 };
 
 static void __init sysctl_init(void)

base-commit: 486855239e38a91d3eae26618ccb7845c1764625
-- 
2.55.0.508.g3f0d502094-goog