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Date:   Wed, 21 Apr 2021 09:33:52 +0200
From:   Oscar Salvador <osalvador@suse.de>
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Peter Zijlstra <peterz@infradead.org>,
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Subject: Re: [External] Re: [PATCH v20 8/9] mm: memory_hotplug: disable
 memmap_on_memory when hugetlb_free_vmemmap enabled
Message-ID: <20210421073346.GA22456@linux>
References: <20210415084005.25049-1-songmuchun@bytedance.com>
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On Wed, Apr 21, 2021 at 11:41:24AM +0800, Muchun Song wrote:
> > Documentation/admin-guide/kernel-parameters.txt already provides an
> > explanation on memory_hotplug.memmap_on_memory parameter that states
> > that the feature cannot be enabled when using hugetlb-vmemmap
> > optimization.
> >
> > Users can always check whether the feature is enabled via
> > /sys/modules/memory_hotplug/parameters/memmap_on_memory.

Heh, I realized this is not completely true.
Users can check whether the feature is __enabled__ by checking the sys fs,
but although it is enabled, it might not be effective.

This might be due to a different number of reasons, vmemmap does not fully
span a PMD, the size we want to add spans more than a single memory block, etc.

That is what

"Note that even when enabled, there are a few cases where the feature is not
 effective."

is supposed to mean.

Anyway, I did not change my opionion on this.

Thanks

-- 
Oscar Salvador
SUSE L3