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

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Date:   Fri, 21 May 2021 10:08:15 +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 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?

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.

-- 
Thanks,

David / dhildenb