linux-kernel - Re: [PATCH v2] kernel/resource: Fix locking in request_free_mem

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Message-ID: <e910441c-73a7-b57e-1330-ead65c4ff412@redhat.com>
Date:   Tue, 30 Mar 2021 11:13:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        daniel.vetter@...ll.ch, dan.j.williams@...el.com,
        gregkh@...uxfoundation.org, jhubbard@...dia.com,
        jglisse@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH v2] kernel/resource: Fix locking in
 request_free_mem_region

On 29.03.21 03:37, Alistair Popple wrote:
> On Friday, 26 March 2021 7:57:51 PM AEDT David Hildenbrand wrote:
>> On 26.03.21 02:20, Alistair Popple wrote:
>>> request_free_mem_region() is used to find an empty range of physical
>>> addresses for hotplugging ZONE_DEVICE memory. It does this by iterating
>>> over the range of possible addresses using region_intersects() to see if
>>> the range is free.
>>
>> Just a high-level question: how does this iteract with memory
>> hot(un)plug? IOW, how defines and manages the "range of possible
>> addresses" ?
> 
> Both the driver and the maximum physical address bits available define the
> range of possible addresses for device private memory. From
> __request_free_mem_region():
> 
> end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 1);
> addr = end - size + 1UL;
> 
> There is no lower address range bound here so it is effectively zero. The code
> will try to allocate the highest possible physical address first and continue
> searching down for a free block. Does that answer your question?

Oh, sorry, the fist time I had a look I got it wrong - I thought (1UL << 
MAX_PHYSMEM_BITS) would be the lower address limit. That looks indeed 
problematic to me.

You might end up reserving an iomem region that could be used e.g., by 
memory hotplug code later. If someone plugs a DIMM or adds memory via 
different approaches (virtio-mem), memory hotplug (via add_memory()) 
would fail.

You never should be touching physical memory area reserved for memory 
hotplug, i.e., via SRAT.

What is the expectation here?

-- 
Thanks,

David / dhildenb