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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <9f385943-4038-f457-c742-30982b8b7d5d@oracle.com>
Date:   Sat, 13 Mar 2021 13:17:22 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.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 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:

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