On Mon, Apr 15, 2024 at 09:52:41AM +0200, Peter Zijlstra wrote:

On Thu, Apr 11, 2024 at 07:00:41PM +0300, Mike Rapoport wrote:

quoted

+/**
+ * enum execmem_type - types of executable memory ranges
+ *
+ * There are several subsystems that allocate executable memory.
+ * Architectures define different restrictions on placement,
+ * permissions, alignment and other parameters for memory that can be used
+ * by these subsystems.
+ * Types in this enum identify subsystems that allocate executable memory
+ * and let architectures define parameters for ranges suitable for
+ * allocations by each subsystem.
+ *
+ * @EXECMEM_DEFAULT: default parameters that would be used for types that
+ * are not explcitly defined.
+ * @EXECMEM_MODULE_TEXT: parameters for module text sections
+ * @EXECMEM_KPROBES: parameters for kprobes
+ * @EXECMEM_FTRACE: parameters for ftrace
+ * @EXECMEM_BPF: parameters for BPF
+ * @EXECMEM_TYPE_MAX:
+ */
+enum execmem_type {
+	EXECMEM_DEFAULT,
+	EXECMEM_MODULE_TEXT = EXECMEM_DEFAULT,
+	EXECMEM_KPROBES,
+	EXECMEM_FTRACE,
+	EXECMEM_BPF,
+	EXECMEM_TYPE_MAX,
+};

Can we please get a break-down of how all these types are actually
different from one another?

I'm thinking some platforms have a tiny immediate space (arm64 comes to
mind) and has less strict placement constraints for some of them?

Yeah, and really I'd *much* rather deal with that in arch code, as I have said
several times.

For arm64 we have two bsaic restrictions: 

1) Direct branches can go +/-128M
   We can expand this range by having direct branches go to PLTs, at a
   performance cost.

2) PREL32 relocations can go +/-2G
   We cannot expand this further.

* We don't need to allocate memory for ftrace. We do not use trampolines.

* Kprobes XOL areas don't care about either of those; we don't place any
  PC-relative instructions in those. Maybe we want to in future.

* Modules care about both; we'd *prefer* to place them within +/-128M of all
  other kernel/module code, but if there's no space we can use PLTs and expand
  that to +/-2G. Since modules can refreence other modules, that ends up
  actually being halved, and modules have to fit within some 2G window that
  also covers the kernel.

* I'm not sure about BPF's requirements; it seems happy doing the same as
  modules.

So if we *must* use a common execmem allocator, what we'd reall want is our own
types, e.g.

	EXECMEM_ANYWHERE
	EXECMEM_NOPLT
	EXECMEM_PREL32

... and then we use those in arch code to implement module_alloc() and friends.

Mark.