linux-kernel - Re: [PATCH v3 10/11] mm/memory_hotplug: Make unregister_memory_block_under

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Message-ID: <20190716084626.GA12394@linux>
Date:   Tue, 16 Jul 2019 10:46:34 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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>,
        Alex Deucher <alexander.deucher@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Mark Brown <broonie@...nel.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>
Subject: Re: [PATCH v3 10/11] mm/memory_hotplug: Make
 unregister_memory_block_under_nodes() never fail

On Mon, Jul 15, 2019 at 01:10:33PM +0200, David Hildenbrand wrote:
> On 01.07.19 12:27, Michal Hocko wrote:
> > On Mon 01-07-19 11:36:44, Oscar Salvador wrote:
> >> On Mon, Jul 01, 2019 at 10:51:44AM +0200, Michal Hocko wrote:
> >>> Yeah, we do not allow to offline multi zone (node) ranges so the current
> >>> code seems to be over engineered.
> >>>
> >>> Anyway, I am wondering why do we have to strictly check for already
> >>> removed nodes links. Is the sysfs code going to complain we we try to
> >>> remove again?
> >>
> >> No, sysfs will silently "fail" if the symlink has already been removed.
> >> At least that is what I saw last time I played with it.
> >>
> >> I guess the question is what if sysfs handling changes in the future
> >> and starts dropping warnings when trying to remove a symlink is not there.
> >> Maybe that is unlikely to happen?
> > 
> > And maybe we handle it then rather than have a static allocation that
> > everybody with hotremove configured has to pay for.
> > 
> 
> So what's the suggestion? Dropping the nodemask_t completely and calling
> sysfs_remove_link() on already potentially removed links?
> 
> Of course, we can also just use mem_blk->nid and rest assured that it
> will never be called for memory blocks belonging to multiple nodes.

Hi David,

While it is easy to construct a scenario where a memblock belongs to multiple
nodes, I have to confess that I yet have not seen that in a real-world scenario.

Given said that, I think that the less risky way is to just drop the nodemask_t
and do not care about calling sysfs_remove_link() for already removed links.
As I said, sysfs_remove_link() will silently fail when it fails to find the
symlink, so I do not think it is a big deal.


-- 
Oscar Salvador
SUSE L3