linux-kernel - Re: [PATCH 5/5] mm/hugetlb: avoid calculating fault_mutex_hash in truncate

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Message-ID: <cc54508f-4feb-1ef6-c33d-5b2c9fcd99d1@huawei.com>
Date:   Mon, 15 Mar 2021 09:50:22 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>, <akpm@...ux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [PATCH 5/5] mm/hugetlb: avoid calculating fault_mutex_hash in
 truncate_op case

On 2021/3/14 5:17, Mike Kravetz wrote:
> On 3/12/21 6:49 PM, Miaohe Lin wrote:
>> Hi:
>> On 2021/3/13 4:03, Mike Kravetz wrote:
>>> On 3/8/21 3:28 AM, Miaohe Lin wrote:
>>>> The fault_mutex hashing overhead can be avoided in truncate_op case because
>>>> page faults can not race with truncation in this routine. So calculate hash
>>>> for fault_mutex only in !truncate_op case to save some cpu cycles.
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>>> ---
>>>>  fs/hugetlbfs/inode.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>>>> index c262566f7c5d..d81f52b87bd7 100644
>>>> --- a/fs/hugetlbfs/inode.c
>>>> +++ b/fs/hugetlbfs/inode.c
>>>> @@ -482,10 +482,9 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>>>>  
>>>>  		for (i = 0; i < pagevec_count(&pvec); ++i) {
>>>>  			struct page *page = pvec.pages[i];
>>>> -			u32 hash;
>>>> +			u32 hash = 0;
>>>
>>> Do we need to initialize hash here?
>>> I would not bring this up normally, but the purpose of the patch is to save
>>> cpu cycles.
>>
>> The hash is initialized here in order to avoid false positive
>> "uninitialized local variable used" warning. Or this is indeed unnecessary?
>>
> 
> Of course.  In this case we know more about usage then the compiler.
> You can add:
> 

I see. Many thanks. Am I supposed to resend the whole v2 patch series ? Or just a single v2 patch with change mentioned above?
Please let me know which is the easiest one for you.

> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
>