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SubjectRe: [PATCH] mm: page_alloc: dump migrate-failed pages only at -EBUSY
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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@kernel.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@kernel.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@kernel.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

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