linux-kernel - Re: [PATCH v2 5/5] mm/hugetlb: avoid calculating fault_mutex_hash in truncate

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Date:   Mon, 15 Mar 2021 20:07:00 -0700
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 v2 5/5] mm/hugetlb: avoid calculating fault_mutex_hash in
 truncate_op case

On 3/15/21 7:27 PM, 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.
> 
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> ---
> v1->v2:
> remove unnecessary initialization for variable hash
> collect Reviewed-by tag from Mike Kravetz

My apologies for not replying sooner and any misunderstanding from my
previous comments.

If the compiler is going to produce a warning because the variable is
not initialized, then we will need to keep the initialization.
Otherwise, this will show up as a build regression.  Ideally, there
would be a modifier which could be used to tell the compiler the
variable will used.  I do not know if such a modifier exists.

The patch can not produce a new warning.  So, if you need to initialize
the variable then do it.  My Reviewed-by still applies.
-- 
Mike Kravetz

> ---
>  fs/hugetlbfs/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index c262566f7c5d..f7ec94bc7337 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -485,7 +485,6 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  			u32 hash;
>  
>  			index = page->index;
> -			hash = hugetlb_fault_mutex_hash(mapping, index);
>  			if (!truncate_op) {
>  				/*
>  				 * Only need to hold the fault mutex in the
> @@ -493,6 +492,7 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>  				 * page faults.  Races are not possible in the
>  				 * case of truncation.
>  				 */
> +				hash = hugetlb_fault_mutex_hash(mapping, index);
>  				mutex_lock(&hugetlb_fault_mutex_table[hash]);
>  			}
>  
>