Re: [PATCH v3 0/5] Allocate memmap from hotadded memory

From: Oscar Salvador <osalvador@suse.de>
To: David Hildenbrand <david@redhat.com>
Cc: akpm@linux-foundation.org, dan.j.williams@intel.com,
	pasha.tatashin@soleen.com, mhocko@suse.com,
	anshuman.khandual@arm.com, Jonathan.Cameron@huawei.com,
	vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/5] Allocate memmap from hotadded memory
Date: Thu, 1 Aug 2019 10:39:01 +0200	[thread overview]
Message-ID: <20190801083856.GA17316@linux> (raw)
In-Reply-To: <1e5776e4-d01e-fe86-57c3-1c3c27aae52f@redhat.com>

On Thu, Aug 01, 2019 at 10:17:23AM +0200, David Hildenbrand wrote:
> I am not yet sure about two things:
> 
> 
> 1. Checking uninitialized pages for PageVmemmap() when onlining. I
> consider this very bad.
> 
> I wonder if it would be better to remember for each memory block the pfn
> offset, which will be used when onlining/offlining.
> 
> I have some patches that convert online_pages() to
> __online_memory_block(struct memory block *mem) - which fits perfect to
> the current user. So taking the offset and processing only these pages
> when onlining would be easy. To do the same for offline_pages(), we
> first have to rework memtrace code. But when offlining, all memmaps have
> already been initialized.

This is true, I did not really like that either, but was one of the things
I came up.
I already have some ideas how to avoid checking the page, I will work on it.

> 2. Setting the Vmemmap pages to the zone of the online type. This would
> mean we would have unmovable data on pages marked to belong to the
> movable zone. I would suggest to always set them to the NORMAL zone when
> onlining - and inititalize the vmemmap of the vmemmap pages directly
> during add_memory() instead.

IMHO, having vmemmap pages in ZONE_MOVABLE do not matter that match.
They are not counted as managed_pages, and they are not show-stopper for
moving all the other data around (migrate), they are just skipped.
Conceptually, they are not pages we can deal with.

I thought they should lay wherever the range lays.
Having said that, I do not oppose to place them in ZONE_NORMAL, as they might
fit there better under the theory that ZONE_NORMAL have memory that might not be
movable/migratable.

As for initializing them in add_memory(), we cannot do that.
First problem is that we first need sparse_mem_map_populate to create
the mapping, and to take the pages from our altmap.

Then, we can access and initialize those pages.
So we cannot do that in add_memory() because that happens before.

And I really think that it fits much better in __add_pages than in add_memory.

Given said that, I would appreciate some comments in patches#3 and patches#4,
specially patch#4.
So I would like to collect some feedback in those before sending a new version.

Thanks David

-- 
Oscar Salvador
SUSE L3