linux-kernel - Re: [PATCH -V5 06/14] hugetlb: Simplify migrate_huge

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Date:	Mon, 09 Apr 2012 14:06:33 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, mgorman@...e.de, dhillf@...il.com,
	aarcange@...hat.com, mhocko@...e.cz, akpm@...ux-foundation.org,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH -V5 06/14] hugetlb: Simplify migrate_huge_page

KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:

> (2012/04/07 3:50), Aneesh Kumar K.V wrote:
>
>> From: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
>> 
>> Since we migrate only one hugepage don't use linked list for passing
>> the page around. Directly pass page that need to be migrated as argument.
>> This also remove the usage page->lru in migrate path.
>> 
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
>
>
> seems good to me. I have one question below.
>
>
>> ---

...... snip ......


>> -	list_add(&hpage->lru, &pagelist);
>> -	ret = migrate_huge_pages(&pagelist, new_page, MPOL_MF_MOVE_ALL, 0,
>> -				true);
>> +	ret = migrate_huge_page(page, new_page, MPOL_MF_MOVE_ALL, 0, true);
>> +	put_page(page);
>>  	if (ret) {
>> -		struct page *page1, *page2;
>> -		list_for_each_entry_safe(page1, page2, &pagelist, lru)
>> -			put_page(page1);
>> -
>>  		pr_info("soft offline: %#lx: migration failed %d, type %lx\n",
>>  			pfn, ret, page->flags);
>> -		if (ret > 0)
>> -			ret = -EIO;   <---------------------------- here
>>  		return ret;
>>  	}
>>  done:
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 51c08a0..d7eb82d 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -929,15 +929,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
>>  	if (anon_vma)
>>  		put_anon_vma(anon_vma);
>>  	unlock_page(hpage);

.... snip .....

>> -
>> -			rc = unmap_and_move_huge_page(get_new_page,
>> -					private, page, pass > 2, offlining,
>> -					mode);
>> -
>> -			switch(rc) {
>> -			case -ENOMEM:
>> -				goto out;
>> -			case -EAGAIN:
>> -				retry++;
>> -				break;
>> -			case 0:
>> -				break;
>> -			default:
>> -				/* Permanent failure */
>> -				nr_failed++;
>> -				break;
>> -			}
>> +			break;
>> +		case 0:
>> +			goto out;
>> +		default:
>> +			rc = -EIO;
>> +			goto out;
>
>
> why -EIO ? Isn't this BUG() ??

I am not sure doing a BUG() for migrate is a good idea. We may want to
return error and let the higher layer handle this. Also as you see in
the two hunks I listed above, default is mapped to -EIO in the current
code. I didn't want to change that.

-aneesh

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