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Subject: Re: [Patch v2] mm/migrate.c: also overwrite error when it is bigger
 than zero
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     akpm@linux-foundation.org, jhubbard@nvidia.com, vbabka@suse.cz,
        cl@linux.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        mhocko@kernel.org
References: <20200119065753.21694-1-richardw.yang@linux.intel.com>
 <20200121015326.GE1567@richard>
From:   Yang Shi <yang.shi@linux.alibaba.com>
Message-ID: <6fc3e936-eed0-eb18-ecd8-36dcc4c51218@linux.alibaba.com>
Date:   Tue, 21 Jan 2020 11:30:03 -0800
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On 1/20/20 5:53 PM, Wei Yang wrote:
> On Sun, Jan 19, 2020 at 02:57:53PM +0800, Wei Yang wrote:
>> If we get here after successfully adding page to list, err would be
>> 1 to indicate the page is queued in the list.
>>
>> Current code has two problems:
>>
>>   * on success, 0 is not returned
>>   * on error, if add_page_for_migratioin() return 1, and the following err1
>>     from do_move_pages_to_node() is set, the err1 is not returned since err
>>     is 1
>>
>> And these behaviors break the user interface.
>>
>> Fixes: e0153fc2c760 ("mm: move_pages: return valid node id in status if the
>> page is already on the target node").
>> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>>
>> ---
>> v2:
>>   * put more words to explain the error case
>> ---
>> mm/migrate.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 86873b6f38a7..430fdccc733e 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1676,7 +1676,7 @@ static int do_pages_move(struct mm_struct *mm, nodemask_t task_nodes,
>> 	err1 = do_move_pages_to_node(mm, &pagelist, current_node);
>> 	if (!err1)
>> 		err1 = store_status(status, start, current_node, i - start);
>> -	if (!err)
>> +	if (err >= 0)
>> 		err = err1;
> Ok, as mentioned by Yang and Michal, only err == 0 means no error.

I think Michal means do_move_pages_to_node() only, 
add_page_for_migration() returns 1 on purpose.

But, actually the syscall may still return > 0 value since err1 might be 
 > 0. Anyway, this is another regression. The patch itself looks correct 
to me.

Acked-by: Yang Shi <yang.shi@linux.alibaba.com>


>
> Sounds this regression should be fixed in another place. Let me send out
> another patch.
>
>> out:
>> 	return err;
>> -- 
>> 2.17.1