linux-kernel - Re: [PATCH v2] x86: mm: Do not use set_{pud,pmd}

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Date:   Tue, 16 Apr 2019 20:30:10 +0000
From:   "Singh, Brijesh" <brijesh.singh@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>
CC:     "Singh, Brijesh" <brijesh.singh@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>
Subject: Re: [PATCH v2] x86: mm: Do not use set_{pud,pmd}_safe when splitting
 the large page



On 4/15/19 11:14 AM, Peter Zijlstra wrote:
> On Mon, Apr 15, 2019 at 08:58:52AM -0700, Dave Hansen wrote:
>> On 4/15/19 7:55 AM, Singh, Brijesh wrote:
>>>   static unsigned long __meminit
>>>   phys_pte_init(pte_t *pte_page, unsigned long paddr, unsigned long paddr_end,
>>> -	      pgprot_t prot)
>>> +	      pgprot_t prot, bool safe)
>>>   {
>>>   	unsigned long pages = 0, paddr_next;
>>>   	unsigned long paddr_last = paddr_end;
>>> @@ -432,7 +463,7 @@ phys_pte_init(pte_t *pte_page, unsigned long paddr, unsigned long paddr_end,
>>>   					     E820_TYPE_RAM) &&
>>>   			    !e820__mapped_any(paddr & PAGE_MASK, paddr_next,
>>>   					     E820_TYPE_RESERVED_KERN))
>>> -				set_pte_safe(pte, __pte(0));
>>> +				__set_pte(pte, __pte(0), safe);
>>>   			continue;
>>>   		}
>>
>> The changelog is great, btw.
>>
>> But, I'm not a big fan of propagating the 'safe' nomenclature.  Could
>> we, at least, call it 'overwrite_safe' or something if we're going to
>> have a variable name?  Or even, 'new_entries_only' or something that
>> actually conveys meaning?
>>
>> Because, just reading it, I always wonder "why do we have an unsafe
>> variant, that's stupid" every time. :)
> 
> s/safe/init/ on the whole thing?
> 

I will update the variable name in v3.

> And maybe even back on the initial _safe functions? Because all of this
> is about initializing page-tables, which is a TLB *safe* operation I
> suppose :-)
> 

Since this particular patch need to pulled into stable hence I am
leaning towards making the _safe function rename after this patch.

>>> +#define DEFINE_ENTRY(type1, type2, safe)			\
>>> +static inline void __set_##type1(type1##_t *arg1,		\
>>> +			type2##_t arg2, bool safe)		\
>>> +{								\
>>> +	if (safe)						\
>>> +		set_##type1##_safe(arg1, arg2);			\
>>> +	else							\
>>> +		set_##type1(arg1, arg2);			\
>>> +}
>>
>> While I appreciate the brevity that these macros allow, I detest their
>> ability to thwart cscope and grep.  I guess it's just one file, but it
>> does make me grumble a bit.
> 
> There is scripts/tags.sh where you can add to regex_c to teach
> cscope/ctags about magic macros.
> 
>> Also, can we do better than "__"?  Aren't these specific to
>> initialization, and only for the kernel?  Maybe we should call them
>> meminit_set_pte() or kern_set_pte() or something so make it totally
>> clear to the reader that they're new.
> 
> set_*_init() and set_*() I suppose.
> 

Will do

>>
>>> -		kernel_physical_mapping_init(__pa(vaddr & pmask),
>>> -					     __pa((vaddr_end & pmask) + psize),
>>> -					     split_page_size_mask);
>>> +		kernel_physical_mapping_change(__pa(vaddr & pmask),
>>> +					       __pa((vaddr_end & pmask) + psize),
>>> +					       split_page_size_mask);
>>
>> BTW, this hunk is really nice the way that the new naming makes it more
>> intuitive what's going on.  My only nit w9uld be that we now have two
>> very similarly-named functions with different TLB-flushing requirements.
>>
>> Could we please include a comment at this site that reminds us that we
>> owe a TLB flush after this?
> 
> Right, a comment would be good. I think my initial proposal had the TLB
> flushing inside, but I see the usage is in a loop, so I appreciate the
> desire to keep the TLB flushing outside.
> 

I've add comment in kernel_physical_mapping_change() definition. I will
add something along that line here as well.

thanks