Re: [PATCH v2 5/5] mm, memory_hotplug: Refactor shrink_zone/pgdat_span

From: osalvador@suse.de
To: Michal Hocko <mhocko@kernel.org>
Cc: akpm@linux-foundation.org, dan.j.williams@intel.com,
	pavel.tatashin@microsoft.com, jglisse@redhat.com,
	Jonathan.Cameron@huawei.com, rafael@kernel.org, david@redhat.com,
	linux-mm@kvack.org, owner-linux-mm@kvack.org
Subject: Re: [PATCH v2 5/5] mm, memory_hotplug: Refactor shrink_zone/pgdat_span
Date: Wed, 28 Nov 2018 12:00:35 +0100	[thread overview]
Message-ID: <ddee6546c35aaada14b196c83f5205e0@suse.de> (raw)
In-Reply-To: <20181128101426.GH6923@dhcp22.suse.cz>

> OK, so what is the difference between memory hotremoving a range 
> withing
> a zone and on the zone boundary? There should be none, yet spanned 
> pages
> do get updated only when we do the later, IIRC? So spanned pages is not
> really all that valuable information. It just tells the
> zone_end-zone_start. Also not what is the semantic of
> spanned_pages for interleaving zones.

Ok, I think I start getting your point.
Yes, spanned_pages are only touched in case we remove the first or the 
last
section of memory range.

So your point is to get rid of shrink_zone_span() and 
shrink_node_span(),
and do not touch spanned_pages at all? (only when the zone is gone or 
the node
goes offline?)

The only thing I am worried about is that by doing that, the system
will account spanned_pages incorrectly.
So, if we remove pages on zone-boundary, neither zone_start_pfn nor
spanned_pages will change.
I did not check yet, but could it be that somewhere we use zone/node's 
spanned_pages
information to compute something?

I mean, do not get me wrong, getting rid of all shrink stuff would be 
great,
it will remove a __lot__ of code and some complexity, but I am not sure 
if
it is totally safe.

>> And if we remove it, would not this give to a user "bad"/confusing
>> information when looking at /proc/zoneinfo?
> 
> Who does use spanned pages for anything really important? It is managed
> pages that people do care about.

Fair enough.