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Thu, 18 Feb 2021 16:53:23 +0000 (UTC) Subject: Re: [PATCH] mm: be more verbose for alloc_contig_range faliures To: Minchan Kim Cc: Michal Hocko , Andrew Morton , linux-mm , LKML , joaodias@google.com References: <20210217163603.429062-1-minchan@kernel.org> <2f167b3c-5f0a-444a-c627-49181fc8fe0d@redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <694c1d6a-d3bc-71ec-7730-e83714e85c25@redhat.com> Date: Thu, 18 Feb 2021 17:53:22 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Stat-Signature: kg7mg7tatjzoza7ciu4jp9a1zry1hinj X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 5090C130 Received-SPF: none (redhat.com>: No applicable sender policy available) receiver=imf04; identity=mailfrom; envelope-from=""; helo=us-smtp-delivery-124.mimecast.com; client-ip=216.205.24.124 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1613667208-930258 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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