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

From: Denys Vlasenko
Date: Sat Sep 17 2016 - 17:09:33 EST

On 09/15/2016 09:04 AM, Ingo Molnar wrote:

* Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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.


+ * 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.