From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux MM <linux-mm@kvack.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Rafael J. Wysocki" <rafael@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Leonardo Bras <leonardo@linux.ibm.com>,
Nathan Lynch <nathanl@linux.ibm.com>,
Allison Randal <allison@lohutok.net>,
Nathan Fontenot <nfont@linux.vnet.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Stephen Rothwell <sfr@canb.auug.org.au>,
Anshuman Khandual <anshuman.khandual@arm.com>,
lantianyu1986@gmail.com,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul
Date: Wed, 22 Jan 2020 11:39:08 +0100 [thread overview]
Message-ID: <a29b49b9-28ad-44fa-6c0b-90cd43902f29@redhat.com> (raw)
In-Reply-To: <20200121120714.GJ29276@dhcp22.suse.cz>
>>> 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
next prev parent reply other threads:[~2020-01-22 10:39 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-17 10:57 [PATCH RFC v1] mm: is_mem_section_removable() overhaul David Hildenbrand
2020-01-17 11:33 ` Michal Hocko
2020-01-17 13:08 ` David Hildenbrand
2020-01-17 14:52 ` Michal Hocko
2020-01-17 14:58 ` David Hildenbrand
2020-01-17 15:29 ` Michal Hocko
2020-01-17 15:54 ` Dan Williams
2020-01-17 16:10 ` David Hildenbrand
2020-01-17 16:57 ` Dan Williams
2020-01-20 7:48 ` Michal Hocko
2020-01-20 9:14 ` David Hildenbrand
2020-01-20 9:20 ` David Hildenbrand
2020-01-21 12:07 ` Michal Hocko
2020-01-22 10:39 ` David Hildenbrand [this message]
2020-01-22 10:42 ` Michal Hocko
2020-01-22 10:54 ` David Hildenbrand
2020-01-22 11:58 ` David Hildenbrand
2020-01-22 16:46 ` Michal Hocko
2020-01-22 18:15 ` David Hildenbrand
2020-01-22 18:38 ` Michal Hocko
2020-01-22 18:46 ` David Hildenbrand
2020-01-22 19:09 ` Michal Hocko
2020-01-22 20:51 ` Dan Williams
2020-01-22 19:01 ` Michal Hocko
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