linux-kernel - Re: [RFC PATCH 6/6] powerpc/kvm: change the condition of identifying hugetlb vm

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Date:	Tue, 7 Jul 2015 16:05:39 +0800
From:	wenwei tao <wenweitaowenwei@...il.com>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Izik Eidus <izik.eidus@...ellosystems.com>, aarcange@...hat.com,
	chrisw@...s-sol.org, Hugh Dickins <hughd@...gle.com>,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, viro@...iv.linux.org.uk, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, kvm@...r.kernel.org,
	kvm-ppc@...r.kernel.org
Subject: Re: [RFC PATCH 6/6] powerpc/kvm: change the condition of identifying
 hugetlb vm

Hi Scott

I understand what you said.

I will use the function 'is_vm_hugetlb_page()' to hide the bit
combinations according to your comments in the next version of patch
set.

But for the situation like below, there isn't an obvious structure
'vma', using 'is_vm_hugetlb_page()' maybe costly or even not possible.
void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start,
                unsigned long end, unsigned long vmflag)
{
    ...

    if (end == TLB_FLUSH_ALL || tlb_flushall_shift == -1
                    || vmflag & VM_HUGETLB) {
        local_flush_tlb();
        goto flush_all;
    }
...
}


Thank you
Wenwei

2015-07-07 5:34 GMT+08:00 Scott Wood <scottwood@...escale.com>:
> On Fri, 2015-07-03 at 16:47 +0800, wenwei tao wrote:
>> Hi Scott
>>
>> Thank you for your comments.
>>
>> Kernel already has that function: is_vm_hugetlb_page() , but the
>> original code didn't use it,
>> in order to keep the coding style of the original code, I didn't use it
>> either.
>>
>> For the sentence like: "vma->vm_flags & VM_HUGETLB" , hiding it behind
>> 'is_vm_hugetlb_page()' is ok,
>> but the sentence like: "vma->vm_flags &
>> (VM_LOCKED|VM_HUGETLB|VM_PFNMAP)" appears in the patch 2/6,
>> is it better to hide the bit combinations behind the
>> is_vm_hugetlb_page() ?  In my patch I just replaced it with
>> "vma->vm_flags & (VM_LOCKED|VM_PFNMAP) ||  (vma->vm_flags &
>> (VM_HUGETLB|VM_MERGEABLE)) == VM_HUGETLB".
>
> If you're going to do non-obvious things with the flags, it should be done in
> one place rather than throughout the code.  Why would you do the above and
> not "vma->vm_flags & (VM_LOCKED | VM_PFNMAP) || is_vm_hugetlb_page(vma)"?
>
> -Scott
>
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