[BUG] RCU Detected Stall in sys_process_vm_writev

[Guoyu Yin <y04609127@gmail.com>]

Hi,

I discovered a kernel crash using the Syzkaller framework, described
as "INFO: rcu detected stall in sys_process_vm_writev". This issue
occurs during the execution of the sys_process_vm_writev system call,
where RCU detects a stall on CPU 0.

From the dmesg log, CPU 3 is stuck trying to acquire a spinlock in the
pgd_free function (arch/x86/mm/pgtable.c:490), leading to the RCU
stall. This is likely caused by spinlock contention triggered by the
page pinning and unpinning logic in sys_process_vm_writev under high
load or abnormal conditions.

I recommend reviewing the page pinning (pin_user_pages_remote) and
unpinning (unpin_user_pages_dirty_lock) logic in
process_vm_rw_single_vec (mm/process_vm_access.c) to ensure it does
not cause prolonged spinlock blocking due to scheduling delays or
resource contention.

This can be reproduced on:

HEAD commit:

fac04efc5c793dccbd07e2d59af9f90b7fc0dca4

report: https://pastebin.com/raw/v7xV4BdD

console output : https://pastebin.com/raw/GfJLqkpf

kernel config: https://pastebin.com/raw/zrj9jd1V

C reproducer : https://pastebin.com/raw/8Mm5f2kh

Best regards,

Guoyu