linux-kernel - Re: [PATCH RFC v1] mm: is_mem_section

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Date:   Wed, 22 Jan 2020 11:39:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Leonardo Bras <leonardo@...ux.ibm.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Anshuman Khandual <anshuman.khandual@....com>,
        lantianyu1986@...il.com,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

>>> Really, the interface is flawed and should have never been merged in the
>>> first place. We cannot simply remove it altogether I am afraid so let's
>>> at least remove the bogus code and pretend that the world is a better
>>> place where everything is removable except the reality sucks...
>>
>> As I expressed already, the interface works as designed/documented and
>> has been used like that for years.
> 
> It seems we do differ in the usefulness though. Using a crappy interface
> for years doesn't make it less crappy. I do realize we cannot remove the
> interface but we can remove issues with the implementation and I dare to
> say that most existing users wouldn't really notice.

Well, at least powerpc-utils (why this interface was introduced) will
notice a) performance wise and b) because more logging output will be
generated (obviously non-offlineable blocks will be tried to offline).

However, it should not break, because we could have had races
before/false positives.

> 
>> I tend to agree that it never should have been merged like that.
>>
>> We have (at least) two places that are racy (with concurrent memory
>> hotplug):
>>
>> 1. /sys/.../memoryX/removable
>> - a) make it always return yes and make the interface useless
>> - b) add proper locking and keep it running as is (e.g., so David can
>>      identify offlineable memory blocks :) ).
>>
>> 2. /sys/.../memoryX/valid_zones
>> - a) always return "none" if the memory is online
>> - b) add proper locking and keep it running as is
>> - c) cache the result ("zone") when a block is onlined (e.g., in
>> mem->zone. If it is NULL, either mixed zones or unknown)
>>
>> At least 2. already scream for a proper device_lock() locking as the
>> mem->state is not stable across the function call.
>>
>> 1a and 2a are the easiest solutions but remove all ways to identify if a
>> memory block could theoretically be offlined - without trying
>> (especially, also to identify the MOVABLE zone).
>>
>> I tend to prefer 1b) and 2c), paired with proper device_lock() locking.
>> We don't affect existing use cases but are able to simplify the code +
>> fix the races.
>>
>> What's your opinion? Any alternatives?
> 
> 1a) and 2c) if you ask me.
> 

I'll look into that all, just might take a little (busy with a lot of
stuff). But after all, it does not seem to be urgent.

1a) will be easy, I'll post a patch soon that we can let rest in -next
for a bit to see if people start to scream out loud.

-- 
Thanks,

David / dhildenb