linux-kernel - Re: [PATCH 2/2 v2] x86/e820: Use much less memory for e820/e820

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Message-ID: <494d4f1a-272f-4ee4-d184-6d6645584012@redhat.com>
Date:   Sat, 17 Sep 2016 23:09:18 +0200
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2 v2] x86/e820: Use much less memory for
 e820/e820_saved, save up to 120k

On 09/15/2016 09:04 AM, Ingo Molnar wrote:
>
> * Denys Vlasenko <dvlasenk@...hat.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.