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Date:   Mon, 30 Jul 2018 16:50:35 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Pavel Tatashin <pasha.tatashin@oracle.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>, gregkh@linuxfoundation.org,
        mingo@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        dan.j.williams@intel.com, jack@suse.cz, mawilcox@microsoft.com,
        jglisse@redhat.com, Souptick Joarder <jrdr.linux@gmail.com>,
        kirill.shutemov@linux.intel.com, Vlastimil Babka <vbabka@suse.cz>,
        osalvador@techadventures.net, yasu.isimatu@gmail.com,
        malat@debian.org, Mel Gorman <mgorman@suse.de>,
        iamjoonsoo.kim@lge.com
Subject: Re: [PATCH v1] mm: inititalize struct pages when adding a section
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On Mon 30-07-18 16:42:27, David Hildenbrand wrote:
> On 30.07.2018 16:10, Michal Hocko wrote:
> > On Mon 30-07-18 15:51:45, David Hildenbrand wrote:
> >> On 30.07.2018 15:30, Pavel Tatashin wrote:
> > [...]
> >>> Hi David,
> >>>
> >>> Have you figured out why we access struct pages during hot-unplug for
> >>> offlined memory? Also, a panic trace would be useful in the patch.
> >>
> >> __remove_pages() needs a zone as of now (e.g. to recalculate if the zone
> >> is contiguous). This zone is taken from the first page of memory to be
> >> removed. If the struct pages are uninitialized that value is random and
> >> we might even get an invalid zone.
> >>
> >> The zone is also used to locate pgdat.
> >>
> >> No stack trace available so far, I'm just reading the code and try to
> >> understand how this whole memory hotplug/unplug machinery works.
> > 
> > Yes this is a mess (evolution of the code called otherwise ;) [1].
> 
> So I guess I should not feel bad if I am having problems understanding
> all the details? ;)
> 
> > Functionality has been just added on top of not very well thought
> > through bases. This is a nice example of it. We are trying to get a zone
> > to 1) special case zone_device 2) recalculate zone state. The first
> > shouldn't be really needed because we should simply rely on altmap.
> > Whether it is used for zone device or not. 2) shouldn't be really needed
> > if the section is offline and we can check that trivially.
> > 
> 
> About 2, I am not sure if this is the case and that easy. To me it looks
> more like remove_pages() fixes up things that should be done in
> offline_pages(). Especially, if the same memory was onlined/offlined to
> different zones we might be in trouble (looking at code on a very high
> level view).

Well, this might be possible. Hotplug remove path was on my todo list
for a long time. I didn't get that far TBH. shrink_zone_span begs for
some attention.

> "if the section is offline and we can check that trivially" is not was
> is being used here. It is "of the section was online and is now offline".

yes.

> Accessing a zone when removing memory sounds very wrong. offline_pages()
> should cleanup everything that online_pages() did.
> 
> Removing memory with pages that are still online should not be allowed.
> And I think this is already enforced via check_memblock_offlined_cb().

yes.
-- 
Michal Hocko
SUSE Labs