Message-ID: <20200117152947.GK19428@dhcp22.suse.cz>
Date:   Fri, 17 Jan 2020 16:29:47 +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 15:58:26, David Hildenbrand wrote:
> On 17.01.20 15:52, Michal Hocko wrote:
> > 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.
> 
> It's a pure speedup. And for that, the interface has been working
> perfectly fine for years?
> 
> >  
> >> 4. lsmem displays/groups by "removable".
> > 
> > Is anybody really using that?
> 
> Well at least I am using that when testing to identify which
> (ZONE_NORMAL!) block I can easily offline/re-online (e.g., to validate
> all the zone shrinking stuff I have been fixing)
> 
> So there is at least one user ;)

Fair enough. But I would argue that there are better ways to do the same
solely for testing purposes. Rather than having a subtly broken code to
maintain.
 
> > 
> >>> 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.
> 
> I can see that, but I can perfectly well understand why - especially
> powerpc - wants a fast way to sense which blocks actually sense to try
> to online.
> 
> The original patch correctly states
>    "which sections of
>     memory are likely to be removable before attempting the potentially
>     expensive operation."
> 
> It works as designed I would say.

Then I would just keep it crippled the same way it has been for years
without anybody noticing.
-- 
Michal Hocko
SUSE Labs

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