On 2021/4/22 15:29, Mike Rapoport wrote:

On Thu, Apr 22, 2021 at 03:00:20PM +0800, Kefeng Wang wrote:

quoted

On 2021/4/21 14:51, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

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 ?

quoted

"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/ (local)

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