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

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Date:   Fri, 17 Jan 2020 15:52:33 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, 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>,
        Dan Williams <dan.j.williams@...el.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Anshuman Khandual <anshuman.khandual@....com>,
        lantianyu1986@...il.com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul

On Fri 17-01-20 14:08:06, David Hildenbrand wrote:
> On 17.01.20 12:33, Michal Hocko wrote:
> > On Fri 17-01-20 11:57:59, David Hildenbrand wrote:
> >> Let's refactor that code. We want to check if we can offline memory
> >> blocks. Add a new function is_mem_section_offlineable() for that and
> >> make it call is_mem_section_offlineable() for each contained section.
> >> Within is_mem_section_offlineable(), add some more sanity checks and
> >> directly bail out if the section contains holes or if it spans multiple
> >> zones.
> > 
> > I didn't read the patch (yet) but I am wondering. If we want to touch
> > this code, can we simply always return true there? I mean whoever
> > depends on this check is racy and the failure can happen even after
> > the sysfs says good to go, right? The check is essentially as expensive
> > as calling the offlining code itself. So the only usecase I can think of
> > is a dumb driver to crawl over blocks and check which is removable and
> > try to hotremove it. But just trying to offline one block after another
> > is essentially going to achieve the same.
> 
> Some thoughts:
> 
> 1. It allows you to check if memory is likely to be offlineable without
> doing expensive locking and trying to isolate pages (meaning:
> zone->lock, mem_hotplug_lock. but also, calling drain_all_pages()
> when isolating)
> 
> 2. There are use cases that want to identify a memory block/DIMM to
> unplug. One example is PPC DLPAR code (see this patch). Going over all
> memory block trying to offline them is an expensive operation.
> 
> 3. powerpc-utils (https://github.com/ibm-power-utilities/powerpc-utils)
> makes use of /sys/.../removable to speed up the search AFAIK.

Well, while I do see those points I am not really sure they are worth
having a broken (by-definition) interface.
 
> 4. lsmem displays/groups by "removable".

Is anybody really using that?

> 5. If "removable=false" then it usually really is not offlineable.
> Of course, there could also be races (free the last unmovable page),
> but it means "don't even try". OTOH, "removable=true" is more racy,
> and gives less guarantees. ("looks okay, feel free to try")

Yeah, but you could be already pessimistic and try movable zones before
other kernel zones.

> > Or does anybody see any reasonable usecase that would break if we did
> > that unconditional behavior?
> 
> If we would return always "true", then the whole reason the
> interface originally was introduced would be "broken" (meaning, less
> performant as you would try to offline any memory block).

I would argue that the whole interface is broken ;). Not the first time
in the kernel development history and not the last time either. What I
am trying to say here is that unless there are _real_ usecases depending
on knowing that something surely is _not_ offlineable then I would just
try to drop the functionality while preserving the interface and see
what happens.
-- 
Michal Hocko
SUSE Labs