Re: [RFC PATCH v2] mm: initialize struct pages reserved by ZONE_DEVICE driver.

[David Hildenbrand <david@redhat.com>]

On 09.09.19 13:53, Dan Williams wrote:
> On Mon, Sep 9, 2019 at 1:11 AM David Hildenbrand <david@redhat.com> wrote:
> [..]
>>>> It seems that SECTION_IS_ONLINE and SECTION_MARKED_PRESENT can be used to
>>>> distinguish uninitialized struct pages if we can apply them to ZONE_DEVICE,
>>>> but that is no longer necessary with this approach.
>>>
>>> Let's take a step back here to understand the issues I am aware of. I
>>> think we should solve this for good now:
>>>
>>> A PFN walker takes a look at a random PFN at a random point in time. It
>>> finds a PFN with SECTION_MARKED_PRESENT && !SECTION_IS_ONLINE. The
>>> options are:
>>>
>>> 1. It is buddy memory (add_memory()) that has not been online yet. The
>>> memmap contains garbage. Don't access.
>>>
>>> 2. It is ZONE_DEVICE memory with a valid memmap. Access it.
>>>
>>> 3. It is ZONE_DEVICE memory with an invalid memmap, because the section
>>> is only partially present: E.g., device starts at offset 64MB within a
>>> section or the device ends at offset 64MB within a section. Don't access it.
>>>
>>> 4. It is ZONE_DEVICE memory with an invalid memmap, because the memmap
>>> was not initialized yet. memmap_init_zone_device() did not yet succeed
>>> after dropping the mem_hotplug lock in mm/memremap.c. Don't access it.
>>>
>>> 5. It is reserved ZONE_DEVICE memory ("pages mapped, but reserved for
>>> driver") with an invalid memmap. Don't access it.
>>>
>>> I can see that your patch tries to make #5 vanish by initializing the
>>> memmap, fair enough. #3 and #4 can't be detected. The PFN walker could
>>> still stumble over uninitialized memmaps.
>>>
>>
>> FWIW, I thinkg having something like pfn_zone_device(), similarly
>> implemented like pfn_zone_device_reserved() could be one solution to
>> most issues.
> 
> I've been thinking of a replacement for PTE_DEVMAP with section-level,
> or sub-section level flags. The section-level flag would still require
> a call to get_dev_pagemap() to validate that the pfn is not section in
> the subsection case which seems to be entirely too much overhead. If
> ZONE_DEVICE is to be a first class citizen in pfn walkers I think it
> would be worth the cost to double the size of subsection_map and to
> identify whether a sub-section is ZONE_DEVICE, or not.
> 
> Thoughts?
> 

I thought about this last week and came up with something like

1. Convert SECTION_IS_ONLINE to SECTION IS_ACTIVE

2. Make pfn_to_online_page() also check that it's not ZONE_DEVICE.
Online pfns are limited to !ZONE_DEVICE.

3. Extend subsection_map to an additional active_map

4. Set SECTION IS_ACTIVE *iff* the whole active_map is set. This keeps
most accesses of pfn_to_online_page() fast. If !SECTION IS_ACTIVE, check
the active_map.

5. Set sub-sections active/unactive in
move_pfn_range_to_zone()/remove_pfn_range_from_zone() - see "[PATCH v4
0/8] mm/memory_hotplug: Shrink zones before removing memory​" for the
latter.

6. Set boot memory properly active (this is a tricky bit :/ ).

However, it turned out too complex for my taste (and limited time to
spend on this), so I abandoned that idea for now. If somebody wants to
pick that up, fine.

-- 

Thanks,

David / dhildenb