linux-kernel - Re: [PATCH] mm: be more verbose for alloc_contig

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Date:   Wed, 17 Feb 2021 18:34:13 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, mhocko@...e.com,
        joaodias@...gle.com
Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures

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@...nel.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