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Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures
To:     Minchan Kim <minchan@kernel.org>
Cc:     Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, joaodias@google.com
References: <20210217163603.429062-1-minchan@kernel.org>
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 <2f167b3c-5f0a-444a-c627-49181fc8fe0d@redhat.com>
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 <dc534e7b-3992-eb37-8399-67258ff03067@redhat.com>
 <YC6aIL67PaYlmeYq@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <694c1d6a-d3bc-71ec-7730-e83714e85c25@redhat.com>
Date:   Thu, 18 Feb 2021 17:53:22 +0100
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On 18.02.21 17:47, Minchan Kim wrote:
> On Thu, Feb 18, 2021 at 05:26:08PM +0100, David Hildenbrand wrote:
>> On 18.02.21 17:19, Minchan Kim wrote:
>>> On Thu, Feb 18, 2021 at 10:43:21AM +0100, David Hildenbrand wrote:
>>>> On 18.02.21 10:35, Michal Hocko wrote:
>>>>> On Thu 18-02-21 10:02:43, David Hildenbrand wrote:
>>>>>> On 18.02.21 09:56, Michal Hocko wrote:
>>>>>>> On Wed 17-02-21 08:36:03, 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.
>>>>>>>
>>>>>>> I agree with David that this has a potential to generate a lot of output
>>>>>>> and it is not really clear whether it is worth it. Page isolation code
>>>>>>> already has REPORT_FAILURE mode which currently used only for the memory
>>>>>>> hotplug because this was just too noisy from the CMA path - d381c54760dc
>>>>>>> ("mm: only report isolation failures when offlining memory").
>>>>>>>
>>>>>>> Maybe migration failures are less likely to fail but still.
>>>>>>
>>>>>> Side note: I really dislike that uncontrolled error reporting on memory
>>>>>> offlining path we have enabled as default. Yeah, it might be useful for
>>>>>> ZONE_MOVABLE in some cases, but otherwise it's just noise.
>>>>>>
>>>>>> Just do a "sudo stress-ng --memhotplug 1" and see the log getting flooded
>>>>>
>>>>> Anyway we can discuss this in a separate thread but I think this is not
>>>>> a representative workload.
>>>>
>>>> Sure, but the essence is "this is noise", and we'll have more noise on
>>>> alloc_contig_range() as we see these calls more frequently. There should be
>>>> an explicit way to enable such *debug* messages.
>>>
>>> alloc_contig_range already has gfp_mask and it respects __GFP_NOWARN.
>>
>> I am not 100% sure it does.
> 
> Oh, it should. Otherwise, let's fix either of caller or
> alloc_contig_range since we have a customer.
> 
> ```
>      ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
>              GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0))
> ```
> 

Oh, interesting. So I certainly want to add that for virtio-mem as well 
- thanks.

And discussed, we should hide the one pr_info... in alloc_contig_range() 
as well.

>>
>>> Why shouldn't people use it if they don't care the failure?
>>
>> Because flooding the log with noise maybe a handful of people on this planet
>> care about is absolutely useless. With the warnings in warn_alloc() people
>> can at least conclude something reasonable.
>>
>>> Semantically, it makes sense to me.
>>>
>>> About the messeage flooding, shouldn't we go with ratelimiting?
>>
>> At least that (see warn_alloc()). But I'd even want to see some other
>> trigger to enable this explicitly on demand.
> 
> No objection.
> 
> How about adding verbose knob under CONFIG_CMA_DEBUGFS with
> alloc_contig_range(..., bool verbose) like start_isolate_page_range?
> 
> If admin turns on the verbose mode under CONFIG_CMA_DEBUGFS,
> cma_alloc will pass alloc_contig_range(...., true).

I'd handle this internally in alloc_contig_range and not pass magic 
flags around. Some kind of debug knob to enable advanced messages 
(exceeding the usual allocation warnings we would usually see).

-- 
Thanks,

David / dhildenb