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Subject: Re: [Patch v2 3/4] mm/migrate.c: check pagelist in
 move_pages_and_store_status()
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, mhocko@suse.com,
        yang.shi@linux.alibaba.com, rientjes@google.com
References: <20200122011647.13636-1-richardw.yang@linux.intel.com>
 <20200122011647.13636-4-richardw.yang@linux.intel.com>
 <f7f70c42-a9a6-6c84-cdb5-230205703ddf@redhat.com>
 <20200129004637.GD12835@richard>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <617f0a47-bb35-822f-5e2a-06b20b386853@redhat.com>
Date:   Wed, 29 Jan 2020 10:36:51 +0100
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 29.01.20 01:46, Wei Yang wrote:
> On Tue, Jan 28, 2020 at 11:21:13AM +0100, David Hildenbrand wrote:
>> On 22.01.20 02:16, Wei Yang wrote:
>>> When pagelist is empty, it is not necessary to do the move and store.
>>> Also it consolidate the empty list check in one place.
>>>
>>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>> Acked-by: Michal Hocko <mhocko@suse.com>
>>> ---
>>>  mm/migrate.c | 9 +++------
>>>  1 file changed, 3 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index a4d3bd6475e1..80d2bba57265 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1499,9 +1499,6 @@ static int do_move_pages_to_node(struct mm_struct *mm,
>>>  {
>>>  	int err;
>>>  
>>> -	if (list_empty(pagelist))
>>> -		return 0;
>>> -
>>>  	err = migrate_pages(pagelist, alloc_new_node_page, NULL, node,
>>>  			MIGRATE_SYNC, MR_SYSCALL);
>>>  	if (err)
>>> @@ -1589,6 +1586,9 @@ static int move_pages_and_store_status(struct mm_struct *mm, int node,
>>>  {
>>>  	int err;
>>>  
>>> +	if (list_empty(pagelist))
>>> +		return 0;
>>> +
>>>  	err = do_move_pages_to_node(mm, pagelist, node);
>>>  	if (err)
>>>  		return err;
>>> @@ -1676,9 +1676,6 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>>>  		current_node = NUMA_NO_NODE;
>>>  	}
>>>  out_flush:
>>> -	if (list_empty(&pagelist))
>>> -		return err;
>>
>>
>> Am I wrong or are you discarding an error here? (err could be !0)
>>
> 
> This is not obvious in this code snippet. If you look into the source code, it
> might help you get the whole picture.
> 
> The following comment says:
> 
>   Make sure we do not overwrite the existing error
> 
> So we didn't eat error here.

My fault, I misread the "if (err >= 0)" (and thought it was a "if (err1
>= 0)")

Makes perfect sense, thanks!

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb