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

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Date:	Fri, 3 Jul 2015 16:47:42 +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

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".

I am a newbie to Linux kernel, do you have any good suggestions on
this situation?

Thank you
Wenwei

2015-07-03 5:49 GMT+08:00 Scott Wood <scottwood@...escale.com>:
> On Wed, 2015-06-10 at 14:27 +0800, Wenwei Tao wrote:
>> Hugetlb VMAs are not mergeable, that means a VMA couldn't have VM_HUGETLB
>> and
>> VM_MERGEABLE been set in the same time. So we use VM_HUGETLB to indicate new
>> mergeable VMAs. Because of that a VMA which has VM_HUGETLB been set is a
>> hugetlb
>> VMA only if it doesn't have VM_MERGEABLE been set in the same time.
>
> Eww.
>
> If you must overload such bit combinations, please hide it behind a
> vm_is_hugetlb() function.
>
> -Scott
>
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