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8b3d838139
We met a kernel crash issue when running stress-ng testing, and the
system crashes when printing the dentry name in dump_mapping().
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
......
The root cause 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.
The page migration issue has been fixed by commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.