linux-kernel - Re: [PATCH 4/7] migrate_pages(): fix failure counting for THP subpages retrying

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Message-ID: <87r13a7n04.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Mon, 27 Jun 2022 12:23:39 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Zi Yan <ziy@...dia.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH 4/7] migrate_pages(): fix failure counting for THP
 subpages retrying

Baolin Wang <baolin.wang@...ux.alibaba.com> writes:

> On 6/27/2022 9:46 AM, Huang, Ying wrote:
>> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
>> 
>>> On 6/24/2022 10:53 AM, Huang Ying wrote:
>>>> If THP is failed to be migrated for -ENOSYS and -ENOMEM, the THP will
>>>> be split into thp_split_pages, and after other pages are migrated,
>>>> pages in thp_split_pages will be migrated with no_subpage_counting ==
>>>> true, because its failure have been counted already.  If some pages in
>>>> thp_split_pages are retried during migration, we should not count
>>>> their failure if no_subpage_counting == true too.  This is done this
>>>> patch to fix the failure counting for THP subpages retrying.
>>>
>>> Good catch. Totally agree with you. It seems we can move the condition
>>> into -EAGAIN case like other cases did?
>>>
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 1ece23d80bc4..491c2d07402b 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1463,7 +1463,7 @@ int migrate_pages(struct list_head *from,
>>> new_page_t get_new_page,
>>>                          case -EAGAIN:
>>>                                  if (is_thp)
>>>                                          thp_retry++;
>>> -                               else
>>> +                               else if (!no_subpage_counting)
>>>                                          retry++;
>>>                                  break;
>> This has another effect except fixing the failure counting.  That
>> is,
>> the split subpages of THP will not be retried for 10 times for -EAGAIN.
>
> Ah, yes.
>
>> TBH, I think that we should do that.  But because this has some behavior
>
> OK. So you afraid that 10 times retry for each subpage of THP will
> waste lots of time?

I just think that it's unnecessary.  We have already regarded the
migration as failed.  And for the worst case, we will try 512 * 10 =
5120 times in total.

>> change, it's better to be done in a separate patch?  Do you have
>> interest to do that on top of this patchset?
>
> Sure. I can send a patch which can be folded into your series. Is this
> OK for you?
>
> By the way, if I do like I said, the patch 4 can be avoided.

I tend to keep both.  [4/7] is just a fix.  You patch will introduce the
behavior change.  And your patch needn't to be folded into this series.
You can send it and push it separately.

Best Regards,
Huang, Ying

>> 
>>> Anyway this patch looks good to me.
>>> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Thanks!
>> Best Regards,
>> Huang, Ying
>> 
>>>> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
>>>> Fixes: 5984fabb6e82 ("mm: move_pages: report the number of non-attempted pages")
>>>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>> Cc: Zi Yan <ziy@...dia.com>
>>>> Cc: Yang Shi <shy828301@...il.com>
>>>> --- >   mm/migrate.c | 3 ++-
>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>>> index 542533e4e3cf..61dab3025a1d 100644
>>>> --- a/mm/migrate.c
>>>> +++ b/mm/migrate.c
>>>> @@ -1477,7 +1477,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>>>    			}
>>>>    		}
>>>>    	}
>>>> -	nr_failed += retry;
>>>> +	if (!no_subpage_counting)
>>>> +		nr_failed += retry;
>>>>    	nr_thp_failed += thp_retry;
>>>>    	/*
>>>>    	 * Try to migrate subpages of fail-to-migrate THPs, no nr_failed