Re: [PATCH 2/2 v2] x86/e820: Use much less memory for e820/e820_saved, save up to 120k

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> On 09/15/2016 09:04 AM, Ingo Molnar wrote:
> >
> >* Denys Vlasenko <dvlasenk@redhat.com> wrote:
> >
> >>The maximum size of e820 map array for EFI systems is defined as
> >>E820_X_MAX (E820MAX + 3 * MAX_NUMNODES).
> >>
> >>In x86_64 defconfig, this ends up with E820_X_MAX = 320, e820 and e820_saved
> >>are 6404 bytes each.
> >>
> >>With larger configs, for example Fedora kernels, E820_X_MAX = 3200, e820 and e820_saved
> >>are 64004 bytes each. Most of this space is wasted. Typical machines have some 20-30
> >>e820 areas at most.
> >>
> >>This patch turns e820 and e820_saved to pointers which initially point to __initdata
> >>tables, of the same size as before.
> >>
> >>At the very end of setup_arch(), when we are done fiddling with these maps,
> >>allocate smaller alloc_bootmem blocks, copy maps there, and change pointers.
> >>
> >>Run-tested.
> >
> >>+/*
> >>+ * Initial e820 and e820_saved are largish __initdata arrays.
> >>+ * Copy them to (usually much smaller) dynamically allocated area.
> >>+ * This is done after all tweaks we ever do to them.
> >>+ */
> >>+__init void e820_reallocate_tables(void)
> >>+{
> >>+	struct e820map *n;
> >>+	int size;
> >>+
> >>+	size = offsetof(struct e820map, map) + sizeof(struct e820entry) * e820->nr_map;
> >>+	n = alloc_bootmem(size);
> >>+	memcpy(n, e820, size);
> >>+	e820 = n;
> >>+
> >>+	size = offsetof(struct e820map, map) + sizeof(struct e820entry) * e820_saved->nr_map;
> >>+	n = alloc_bootmem(size);
> >>+	memcpy(n, e820_saved, size);
> >>+	e820_saved = n;
> >>+}
> >
> >Ok, this makes me quite nervous, could you please split this into two patches so
> >that any fails can be nicely bisected to?
> 
> No problem.
> 
> >First patch only does the pointerization changes with a trivial placeholder
> >structure (full size, static allocated), second patch does all the dangerous bits
> >such as changing it to __initdata, allocating and copying over bits.
> >
> >Also, could we please also add some minimal debugging facility to make sure the
> >memory table does not get extended after it's been reallocated?
> 
> I have another idea: run e820_reallocate_tables() later, just before
> we free __init and __initdata. Then e820 tables _can't_ be_ changed -
> all functions which do that are __init functions.
> 
> Will test this now, and send a patchset.

Could we also mark it __ro_after_init?

Thanks,

	Ingo