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

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Date:   Tue, 8 Mar 2022 09:32:39 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Miaohe Lin <linmiaohe@...wei.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 3/7/2022 8:20 PM, Miaohe Lin wrote:
> 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?

Yes, that's possible. Thanks for your explanation. It will be better if 
you can copy the possible scenario description to the commit log to help 
other understand the issue.

Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>