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SubjectRe: [PATCH] mm: be more verbose for alloc_contig_range faliures
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On 17.02.21 18:26, Minchan Kim wrote:
> On Wed, Feb 17, 2021 at 05:51:27PM +0100, David Hildenbrand wrote:
>> On 17.02.21 17:36, Minchan Kim wrote:
>>> alloc_contig_range is usually used on cma area or movable zone.
>>> It's critical if the page migration fails on those areas so
>>> dump more debugging message like memory_hotplug unless user
>>> specifiy __GFP_NOWARN.
>>>
>>> Signed-off-by: Minchan Kim <minchan@kernel.org>
>>> ---
>>> mm/page_alloc.c | 16 +++++++++++++++-
>>> 1 file changed, 15 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index 0b55c9c95364..67f3ee3a1528 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -8486,6 +8486,15 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>>> NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
>>> }
>>> if (ret < 0) {
>>> + if (!(cc->gfp_mask & __GFP_NOWARN)) {
>>> + struct page *page;
>>> +
>>> + list_for_each_entry(page, &cc->migratepages, lru) {
>>> + pr_warn("migrating pfn %lx failed ret:%d ",
>>> + page_to_pfn(page), ret);
>>> + dump_page(page, "migration failure");
>>> + }
>>
>> This can create *a lot* of noise. For example, until huge pages are actually
>> considered, we will choke on each end every huge page - and might do so over
>> and over again.
>
> I am not familiar with huge page status at this moment but why couldn't
> they use __GFP_NOWARN if they are supposed to fail frequently?

any alloc_contig_range() user will fail on hugetlbfs pages right now
when they are placed into CMA/ZONE_MOVABLE. Oscar is working on that
upstream.

>
>>
>> This might be helpful for debugging, but is unacceptable for production
>> systems for now I think. Maybe for now, do it based on CONFIG_DEBUG_VM.
>
> If it's due to huge page you mentioned above and caller passes
> __GFP_NOWARN in that case, couldn't we enable always-on?

It would make sense to add that for virito-mem when calling
alloc_contig_range(). For now I didn't do so, because there were not
that many messages yet - alloc_contig_range() essentially didn't
understand __GFP_NOWARN.

We should then also stop printing the "PFNs busy ..." part from
alloc_contig_range() with __GFP_NOWARN.

>
> Actually, I am targeting cma allocation failure, which should
> be rather rare compared to other call sites but critical to fail.
> If it's concern to emit too many warning message, I will scope
> down for site for only cma allocation.

If you add "__GFP_NOWARN" when !ZONE_MOVABLE, how would you ever print
something for CMA? What am I missing? CMA is usually not on ZONE_MOVABLE.

--
Thanks,

David / dhildenb

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