Re: [PATCH v2 0/4] arm64: drop pfn_valid_within() and simplify pfn_valid()

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2021/4/22 15:29, Mike Rapoport wrote:
> On Thu, Apr 22, 2021 at 03:00:20PM +0800, Kefeng Wang wrote:
>> On 2021/4/21 14:51, Mike Rapoport wrote:
>>> From: Mike Rapoport <rppt@linux.ibm.com>
>>>
>>> Hi,
>>>
>>> These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
>>> pfn_valid_within() to 1.
>>>
>>> The idea is to mark NOMAP pages as reserved in the memory map and restore
>>> the intended semantics of pfn_valid() to designate availability of struct
>>> page for a pfn.
>>>
>>> With this the core mm will be able to cope with the fact that it cannot use
>>> NOMAP pages and the holes created by NOMAP ranges within MAX_ORDER blocks
>>> will be treated correctly even without the need for pfn_valid_within.
>>>
>>> The patches are only boot tested on qemu-system-aarch64 so I'd really
>>> appreciate memory stress tests on real hardware.
>>>
>>> If this actually works we'll be one step closer to drop custom pfn_valid()
>>> on arm64 altogether.
>> Hi Mike，I have a question, without HOLES_IN_ZONE, the pfn_valid_within() in
>> move_freepages_block()->move_freepages()
>> will be optimized, if there are holes in zone, the 'struce page'(memory map)
>> for pfn range of hole will be free by
>> free_memmap(), and then the page traverse in the zone(with holes) from
>> move_freepages() will meet the wrong page，
>> then it could panic at PageLRU(page) test, check link[1],
> First, HOLES_IN_ZONE name us hugely misleading, this configuration option
> has nothing to to with memory holes, but rather it is there to deal with
> holes or undefined struct pages in the memory map, when these holes can be
> inside a MAX_ORDER_NR_PAGES region.
>
> In general pfn walkers use pfn_valid() and pfn_valid_within() to avoid
> accessing *missing* struct pages, like those that are freed at
> free_memmap(). But on arm64 these tests also filter out the nomap entries
> because their struct pages are not initialized.
>
> The panic you refer to happened because there was an uninitialized struct
> page in the middle of MAX_ORDER_NR_PAGES region because it corresponded to
> nomap memory.
>
> With these changes I make sure that such pages will be properly initialized
> as PageReserved and the pfn walkers will be able to rely on the memory map.
>
> Note also, that free_memmap() aligns the parts being freed on MAX_ORDER
> boundaries, so there will be no missing parts in the memory map within a
> MAX_ORDER_NR_PAGES region.

Ok, thanks, we met a same panic like the link on arm32(without 
HOLES_IN_ZONE),

the scheme for arm64 could be suit for arm32, right?  I will try the 
patchset with

some changes on arm32 and give some feedback.

Again, the stupid question, where will mark the region of memblock with

MEMBLOCK_NOMAP flag ?


>   
>> "The idea is to mark NOMAP pages as reserved in the memory map", I see the
>> patch2 check memblock_is_nomap() in memory region
>> of memblock, but it seems that memblock_mark_nomap() is not called(maybe I
>> missed), then memmap_init_reserved_pages() won't
>> work, so should the HOLES_IN_ZONE still be needed for generic mm code?
>>
>> [1] https://lore.kernel.org/linux-arm-kernel/541193a6-2bce-f042-5bb2-88913d5f1047@arm.com/
>>

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