linux-kernel - Re: [PATCH v3 09/11] mm/memory_hotplug: Remove memory block devices before arch_remove

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Message-ID: <54a2f873-374e-b132-ae0f-4924a7e332c0@redhat.com>
Date:   Mon, 15 Jul 2019 12:58:22 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, akpm@...ux-foundation.org,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "mike.travis@....com" <mike.travis@....com>,
        Andrew Banman <andrew.banman@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Mark Brown <broonie@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Oscar Salvador <osalvador@...e.de>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Arun KS <arunks@...eaurora.org>,
        Mathieu Malaterre <malat@...ian.org>
Subject: Re: [PATCH v3 09/11] mm/memory_hotplug: Remove memory block devices
 before arch_remove_memory()

On 01.07.19 10:41, Michal Hocko wrote:
> On Mon 27-05-19 13:11:50, David Hildenbrand wrote:
>> Let's factor out removing of memory block devices, which is only
>> necessary for memory added via add_memory() and friends that created
>> memory block devices. Remove the devices before calling
>> arch_remove_memory().
>>
>> This finishes factoring out memory block device handling from
>> arch_add_memory() and arch_remove_memory().
> 
> OK, this makes sense again. Just a nit. Calling find_memory_block_by_id
> for each memory block looks a bit suboptimal, especially when we are
> removing consequent physical memblocks. I have to confess that I do not
> know how expensive is the search and I also expect that there won't be
> that many memblocks in the removed range anyway as large setups have
> large memblocks.
> 

The devices are not allocated sequentially, so there is no easy way to
look them up.

There is a comment for find_memory_block():

"For now, we have a linear search to go find the appropriate
memory_block corresponding to a particular phys_index. If this gets to
be a real problem, we can always use a radix tree or something here."

So if this becomes a problem, we need a separate data structure to speed
up the lookup. (IOW, this was already the same in the old code)

Thanks!

-- 

Thanks,

David / dhildenb