linux-kernel - Re: [PATCH] powerpc: mm: radix_tlb: rearrange the if-else block

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Message-ID: <90ea33c6-2e93-ea19-3052-90e15979578f@csgroup.eu>
Date:   Fri, 26 Nov 2021 17:25:04 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Nathan Chancellor <nathan@...nel.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     Anders Roxell <anders.roxell@...aro.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>, llvm@...ts.linux.dev,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] powerpc: mm: radix_tlb: rearrange the if-else block



Le 26/11/2021 à 16:46, Nathan Chancellor a écrit :
> On Fri, Nov 26, 2021 at 02:59:29PM +0100, Arnd Bergmann wrote:
>> On Fri, Nov 26, 2021 at 2:43 PM Christophe Leroy
>> <christophe.leroy@...roup.eu> wrote:
>>> Le 25/11/2021 à 16:44, Anders Roxell a écrit :
>>> Can't you fix CLANG instead :) ?
>>>
>>> Or just add an else to the IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) that
>>> sets hstart and hend to 0 ?
>>
>> That doesn't sound any less risky than duplicating the code, it can lead to
>> incorrect changes just as easily if a patch ends up actually flushing at the
>> wrong address, and the compiler fails to complain because of the bogus
>> initialization.
>>
>>> Or just put hstart and hend calculation outside the IS_ENABLED() ? After
>>> all GCC should drop the calculation when not used.
>>
>> I like this one. I'm still unsure how clang can get so confused about whether
>> the variables are initialized or not, usually it handles this much better than
>> gcc. My best guess is that one of the memory clobbers makes it conclude
>> that 'hflush' can be true when it gets written to by an inline asm.
> 
> As far as I am aware, clang's analysis does not evaluate variables when
> generating a control flow graph and using that for static analysis:
> 
> https://godbolt.org/z/PdGxoq9j7
> 
> Based on the control flow graph, it knows that hstart and hend are
> uninitialized because IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) gets
> expanded to 0 by the preprocessor but it does not seem like it can piece
> together that hflush's value of false is only changed to true under the
> now 'if (0) {' branch, meaning that all the calls to __tlbiel_va_range()
> never get evaluated. That may or may not be easy to fix in clang but we
> run into issues like this so infrequently.
> 
> At any rate, the below diff works for me.
> 
> Cheers,
> Nathan
> 
> diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c
> index 7724af19ed7e..156a631df976 100644
> --- a/arch/powerpc/mm/book3s64/radix_tlb.c
> +++ b/arch/powerpc/mm/book3s64/radix_tlb.c
> @@ -1174,12 +1174,10 @@ static inline void __radix__flush_tlb_range(struct mm_struct *mm,
>   		bool hflush = false;
>   		unsigned long hstart, hend;
>   
> -		if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> -			hstart = (start + PMD_SIZE - 1) & PMD_MASK;
> -			hend = end & PMD_MASK;
> -			if (hstart < hend)
> -				hflush = true;
> -		}
> +		hstart = (start + PMD_SIZE - 1) & PMD_MASK;
> +		hend = end & PMD_MASK;
> +		if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && hstart < hend)
> +			hflush = true;

Yes I like that much better.

Maybe even better with

	hflush = IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && hstart < hend;

(And remove default false value at declaration).

>   
>   		if (type == FLUSH_TYPE_LOCAL) {
>   			asm volatile("ptesync": : :"memory");
>