mm: migrate: fix getting incorrect page mapping during page migration

In the Linux kernel, the following vulnerability has been resolved:

mm: migrate: fix getting incorrect page mapping during page migration

When running stress-ng testing, we found below kernel crash after a few hours:

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000

pc : dentry_name+0xd8/0x224

lr : pointer+0x22c/0x370

sp : ffff800025f134c0

......

Call trace:

dentry_name+0xd8/0x224

pointer+0x22c/0x370

vsnprintf+0x1ec/0x730

vscnprintf+0x2c/0x60

vprintk_store+0x70/0x234

vprintk_emit+0xe0/0x24c

vprintk_default+0x3c/0x44

vprintk_func+0x84/0x2d0

printk+0x64/0x88

__dump_page+0x52c/0x530

dump_page+0x14/0x20

set_migratetype_isolate+0x110/0x224

start_isolate_page_range+0xc4/0x20c

offline_pages+0x124/0x474

memory_block_offline+0x44/0xf4

memory_subsys_offline+0x3c/0x70

device_offline+0xf0/0x120

......

After analyzing the vmcore, I found this issue is caused by page migration.

The scenario is that, one thread is doing page migration, and we will use the

target page's ->mapping field to save 'anon_vma' pointer between page unmap and

page move, and now the target page is locked and refcount is 1.

Currently, there is another stress-ng thread performing memory hotplug,

attempting to offline the target page that is being migrated. It discovers that

the refcount of this target page is 1, preventing the offline operation, thus

proceeding to dump the page. However, page_mapping() of the target page may

return an incorrect file mapping to crash the system in dump_mapping(), since

the target page->mapping only saves 'anon_vma' pointer without setting

PAGE_MAPPING_ANON flag.

There are seveval ways to fix this issue:

(1) Setting the PAGE_MAPPING_ANON flag for target page's ->mapping when saving

'anon_vma', but this can confuse PageAnon() for PFN walkers, since the target

page has not built mappings yet.

(2) Getting the page lock to call page_mapping() in __dump_page() to avoid crashing

the system, however, there are still some PFN walkers that call page_mapping()

without holding the page lock, such as compaction.

(3) Using target page->private field to save the 'anon_vma' pointer and 2 bits

page state, just as page->mapping records an anonymous page, which can remove

the page_mapping() impact for PFN walkers and also seems a simple way.

So I choose option 3 to fix this issue, and this can also fix other potential

issues for PFN walkers, such as compaction.