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

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Message-ID: <d40df812-7b02-78fd-65d4-41bbcc9d4c6d@huawei.com>
Date:   Sat, 13 Mar 2021 10:49:26 +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

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?

Many thanks for review.

>