linux-kernel - Re: [PATCH 13/16] mm/migration: return errno when isolate_huge

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Date:   Mon, 7 Mar 2022 20:20:16 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        <akpm@...ux-foundation.org>
CC:     <mike.kravetz@...cle.com>, <shy828301@...il.com>,
        <willy@...radead.org>, <ying.huang@...el.com>, <ziy@...dia.com>,
        <minchan@...nel.org>, <apopple@...dia.com>,
        <ave.hansen@...ux.intel.com>, <o451686892@...il.com>,
        <almasrymina@...gle.com>, <jhubbard@...dia.com>,
        <rcampbell@...dia.com>, <peterx@...hat.com>,
        <naoya.horiguchi@....com>, <mhocko@...e.com>, <riel@...hat.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 13/16] mm/migration: return errno when isolate_huge_page
 failed

On 2022/3/7 10:14, Baolin Wang wrote:
> Hi Miaohe,
> 
> On 3/4/2022 5:34 PM, Miaohe Lin wrote:
>> We should return errno (-EBUSY here) when failed to isolate the huge page
>> rather than always return 1 which could confuse the user.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>> ---
>>   mm/migrate.c | 6 ++----
>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 6c2dfed2ddb8..279940c0c064 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1618,10 +1618,8 @@ static int add_page_for_migration(struct mm_struct *mm, unsigned long addr,
>>           goto out_putpage;
>>         if (PageHuge(page)) {
>> -        if (PageHead(page)) {
>> -            isolate_huge_page(page, pagelist);
>> -            err = 1;
>> -        }
>> +        if (PageHead(page))
>> +            err = isolate_huge_page(page, pagelist) ? 1 : -EBUSY;
> 
> Could you elaborate on which case the huge page isolation can be failed in this case? Or you met a real problem? Cause now we've got this head huge page refcnt, I can not see why we'll fail to isolate this huge page.

IIUC, this could happen when hugepage is under migration which cleared HPageMigratable. Page refcnt cannot
prevent isolate_huge_page from happening. Or am I miss something?

Many thanks.

> .