linux-kernel - Re: [PATCH] x86/mm/pat: Do a small optimization when dump PAT memtype list

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Date:	Fri, 24 Jul 2015 10:40:00 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
	"bp@...e.de" <bp@...e.de>, "Kani, Toshimitsu" <toshi.kani@...com>,
	"jgross@...e.com" <jgross@...e.com>,
	"mcgrof@...e.com" <mcgrof@...e.com>,
	"mnipxh@....com" <mnipxh@....com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH] x86/mm/pat: Do a small optimization when dump PAT memtype
  list

hi, Elliott
	thanks for your reply. :)

On 2015年07月23日 22:53, Elliott, Robert (Server Storage) wrote:
>> -----Original Message-----
>> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-
>> owner@...r.kernel.org] On Behalf Of Pan Xinhui
>> Sent: Thursday, July 23, 2015 4:54 AM
>> To: linux-kernel@...r.kernel.org
>> Subject: [PATCH] x86/mm/pat: Do a small optimization when dump PAT memtype
>> list
> ...
>> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
>> index 268b2c8..6302119 100644
>> --- a/arch/x86/mm/pat.c
>> +++ b/arch/x86/mm/pat.c
>> @@ -1001,45 +1001,42 @@ EXPORT_SYMBOL_GPL(pgprot_writethrough);
>>
>>  #if defined(CONFIG_DEBUG_FS) && defined(CONFIG_X86_PAT)
>>
>> -static struct memtype *memtype_get_idx(loff_t pos)
>> +static struct memtype *memtype_get_idx(struct memtype *entry, loff_t pos)
>>  {
>> -     struct memtype *print_entry;
>>       int ret;
>>
>> -     print_entry  = kzalloc(sizeof(struct memtype), GFP_KERNEL);
>> -     if (!print_entry)
>> -             return NULL;
>> -
>>       spin_lock(&memtype_lock);
>> -     ret = rbt_memtype_copy_nth_element(print_entry, pos);
>> +     ret = rbt_memtype_copy_nth_element(entry, pos);
>>       spin_unlock(&memtype_lock);
>>
>> -     if (!ret) {
>> -             return print_entry;
>> -     } else {
>> -             kfree(print_entry);
>> -             return NULL;
>> -     }
>> +     return ret ? NULL : entry;
>>  }
>>
> ...
>>  static void memtype_seq_stop(struct seq_file *seq, void *v)
>>  {
>> +     kfree(seq->private);
>>  }
>>
> 
> Consider adding
>         seq->private = NULL;
> so the stale pointer isn't left around.  There's probably not
> much risk of accessing it, but NULL is safer in case it is.
> 
I know exactly what are you worrying about.
start and stop callback is still not the best place to alloc/free the *entry*. you are worrying about that some codes might touch it.

seq_file.c has offered seq_open_private and seq_release_private. we can make use of them. :)
I will work out patch v2, this time I will Cc you. I am very happy if you could review it at any free time. :)

thanks
xinhui
> ---
> Robert Elliott, HP Server Storage
> 
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