linux-kernel - Re: [Resend PATCH] x86/e820: break the loop when the region is less then current region

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Date:   Sat, 28 Jan 2017 02:09:03 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Resend PATCH] x86/e820: break the loop when the region is less
 then current region

Hi, Ingo

Glad to see your comments.

On Fri, Jan 27, 2017 at 09:47:23AM +0100, Ingo Molnar wrote:
>
>* Wei Yang <richard.weiyang@...il.com> wrote:
>
>> e820_all_mapped() iterates the e820 table to check whether a region is
>> mapped or not. Since the e820 table is sorted, when the region is less than
>> the current region, no need to continue the iteration.
>> 
>> The patch breaks the loop accordingly.
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@...il.com>
>> ---
>>  arch/x86/kernel/e820.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
>> index 90e8dde..f4fb197 100644
>> --- a/arch/x86/kernel/e820.c
>> +++ b/arch/x86/kernel/e820.c
>> @@ -86,10 +86,16 @@ int __init e820_all_mapped(u64 start, u64 end, unsigned type)
>>  	for (i = 0; i < e820->nr_map; i++) {
>>  		struct e820entry *ei = &e820->map[i];
>>  
>> +		/* Since the e820 table is sorted, when the region is less
>> +		 * than the current region, break it.
>> +		 */
>> +		if (ei->addr >= end)
>> +			break;
>
>Please have a look at the relevant sections in Documentation/CodingStyle. (And 
>yes, this file violates it in a number of ways, but that's no reason to add to the 
>mess.)
>

I took a look in the Documentation/CodingStyle, looks the comment style is not
the preferred one. While I have checked this by scripts/checkpatch.pl, which
prompts 0 error and 0 warning, so I thought it is fine.

I would change this if you like.

>But, more importantly, the reason I have not applied the patch before is that 
>while it's true that the e820 map is _eventually_ sorted, have you made certain 
>that all calls to e820_all_mapped() are done when the map is already sorted?
>

I think so, if not those caller really need to make sure not to do this at
that moment.

The e820_all_mapped() can't work well if it is not sorted, even without this
change.

For example, we have two ranges [0x1000, 0x1FFF] and [0x2000, 0x2FFF] but they
are not sorted in e820. Then a range [0x1500, 0x2500] would be judge not all
mapped, which actually is in range [0x1000, 0x2FFF].

BTW, I went through all the callers, and most of them are called from
subsys_init() or arch_initcall() which is after setup_memory_map() called in
setup_arch().

And two of them are called by e820_add_kernel_range() and
efi_reserve_boot_services(), and both are after setup_memory_map().

>Thanks,
>
>	Ingo

-- 
Wei Yang
Help you, Help me

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