linux-kernel - Re: [PATCH] mm: page_alloc: dump migrate-failed pages only at -EBUSY

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Date:   Sun, 23 May 2021 14:06:20 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        John Dias <joaodias@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: page_alloc: dump migrate-failed pages only at -EBUSY

On 21.05.21 19:39, Minchan Kim wrote:
> On Fri, May 21, 2021 at 10:08:15AM +0200, David Hildenbrand wrote:
>> On 20.05.21 22:51, Minchan Kim wrote:
>>> On Thu, May 20, 2021 at 09:28:09PM +0200, David Hildenbrand wrote:
>>>> Minchan Kim <minchan@...nel.org> schrieb am Do. 20. Mai 2021 um 21:20:
>>>>
>>>>> On Wed, May 19, 2021 at 02:33:41PM -0700, Minchan Kim wrote:
>>>>>> alloc_contig_dump_pages aims for helping debugging page migration
>>>>>> failure by page refcount mismatch or something else of page itself
>>>>>> from migration handler function. However, in -ENOMEM case, there is
>>>>>> nothing to get clue from page descriptor information so just
>>>>>> dump pages only when -EBUSY happens.
>>>>>>
>>>>>> Signed-off-by: Minchan Kim <minchan@...nel.org>
>>>>>> ---
>>>>>>    mm/page_alloc.c | 3 ++-
>>>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>>>> index 3100fcb08500..c0a2971dc755 100644
>>>>>> --- a/mm/page_alloc.c
>>>>>> +++ b/mm/page_alloc.c
>>>>>> @@ -8760,7 +8760,8 @@ static int __alloc_contig_migrate_range(struct
>>>>> compact_control *cc,
>>>>>>
>>>>>>         lru_cache_enable();
>>>>>>         if (ret < 0) {
>>>>>> -             alloc_contig_dump_pages(&cc->migratepages);
>>>>>> +             if (ret == -EBUSY)
>>>>>> +                     alloc_contig_dump_pages(&cc->migratepages);
>>>>>>                 putback_movable_pages(&cc->migratepages);
>>>>>>                 return ret;
>>>>>>         }
>>>>>> --
>>>>>> 2.31.1.751.gd2f1c929bd-goog
>>>>>>
>>>>>
>>>>> Resend with a little modifying description.
>>>>>
>>>>>   From c5a2fea291cf46079b87cc9ac9a25fc7f819d0fd Mon Sep 17 00:00:00 2001
>>>>> From: Minchan Kim <minchan@...nel.org>
>>>>> Date: Wed, 19 May 2021 14:22:18 -0700
>>>>> Subject: [PATCH] mm: page_alloc: dump migrate-failed pages only at -EBUSY
>>>>>
>>>>> alloc_contig_dump_pages aims for helping debugging page migration
>>>>> failure by elevated page refcount compared to expected_count.
>>>>> (for the detail, please look at migrate_page_move_mapping)
>>>>>
>>>>> However, -ENOMEM is just the case that system is under memory
>>>>> pressure state, not relevant with page refcount at all. Thus,
>>>>> the dumping page list is not helpful for the debugging point of view.
>>>>>
>>>>
>>>> what about -ENOMEM when migrating empty/free huge pages? I think there is
>>>> value in having the pages dumped to identify something like that. And it
>>>> doesn‘t require heavy memory pressure to fail allocating a huge page.
>>>>
>>>
>>> -ENOMEM means there is no memory to alloate destination page.
>>> How could it help dumping source pages in those case from dump_page
>>> content point of view?
>>
>> You would spot a huge page in the source list (usually at first position)
>> without any obvious migration blockers I assume?
> 
> It was not a huge page case.
> 
>>
>> I'm wondering, did you actually run into this being suboptimal? If it's a
>> real problem dumping too many stuff when running into -ENOMEM, fine with me.
>> If it's a theoretical issue, I'd prefer to just keep it simple as is.
> 
> That's exactly what I encountered. With -ENOMEM, it dumped bunch of
> pages on migratepages list. It was just useless with just consuming
> logbuffer since there are nothing much to investigate with dumping
> source pages.
> 

Fine with me, then

Reviewed-by: David Hildenbrand <david@...hat.com>

Thanks!

-- 
Thanks,

David / dhildenb