Re: [RFC PATCH 8/8] HACK: mm: memory_hotplug: Drop memblock_phys_free() call in try_remove_memory()

From: Mike Rapoport <rppt@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Dan Williams <dan.j.williams@intel.com>,
	linux-cxl@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	Sudeep Holla <sudeep.holla@arm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Will Deacon <will@kernel.org>, Jia He <justin.he@arm.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	linuxarm@huawei.com, catalin.marinas@arm.com,
	Anshuman.Khandual@arm.com,
	Yuquan Wang <wangyuquan1236@phytium.com.cn>,
	Oscar Salvador <osalvador@suse.de>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	James Morse <james.morse@arm.com>
Subject: Re: [RFC PATCH 8/8] HACK: mm: memory_hotplug: Drop memblock_phys_free() call in try_remove_memory()
Date: Wed, 5 Jun 2024 11:00:03 +0300	[thread overview]
Message-ID: <ZmAbA-kXEfKBKj4-@kernel.org> (raw)
In-Reply-To: <900bd9b6-a788-4c71-9b43-aff4855ba234@redhat.com>

On Tue, Jun 04, 2024 at 11:39:27AM +0200, David Hildenbrand wrote:
> On 04.06.24 11:35, Mike Rapoport wrote:
> > On Mon, Jun 03, 2024 at 10:53:03PM +0200, David Hildenbrand wrote:
> > > On 03.06.24 12:43, Mike Rapoport wrote:
> > > > On Mon, Jun 03, 2024 at 11:14:00AM +0200, David Hildenbrand wrote:
> > > 
> > > > The commit that added memblock_free() at the first place (f9126ab9241f
> > > > ("memory-hotplug: fix wrong edge when hot add a new node")) does not really
> > > > describe why that was required :(
> > > > 
> > > > But at a quick glance it looks completely spurious.
> > > 
> > > There are more details [1] but I also did not figure out why the
> > > memblock_free() was really required to resolve that issue.
> > > 
> > > [1] https://marc.info/?l=linux-kernel&m=142961156129456&w=2
> > The tinkering with memblock there and in f9126ab9241f seem bogus in the
> > context of memory hotplug on x86.
> > 
> > I believe that dropping that memblock_phys_free() is right thing to do
> > regardless of this series. There's no corresponding memblock_alloc() and it
> > was added as part of a fix for hotunplug on x86 that anyway had memblock
> > discarded at that point.
> 
> So when we re-add that memory, we might have still ranges as "reserved".

I don't see how anything can become reserved on the hotplug path unless
hotplug is possible before mm_core_init().
There are no memblock_reserve() calls in memory_hotplug.c, no memblock
allocations possible after mm is inited, and even if memblock_add() will
need to allocate memory that will be done via slab.

> It does sound weird, but you're the boss :)

Nah, it's mm/memory_hotplug.c, so you are :)

But I can send a patch anyway :)

> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.