Page fault while link_path_walk for path_len > 4060 bytes

[ankijain@codeaurora.org]

Hi

We are facing a issue while creating the directories up-to maximum 
depth.
While doing this, page fault occurs some times if PATH_LEN is near to 
page boundary.
During hash_name() calculation inside link_path_walk(), 
load_unaligned_zeropad() is causing page fault as it tries to access 
next page data(which is not mapped).
We have already taken a lock(rcu_read_lock) in path_init() and as a part 
of page fault it checks if process can sleep (might_sleep), which 
results in panic as rcu_read_lock is already taken in path_init().

One observation : Panic happens only when page(which contain the 
path_name and length > 4060) is the last page of the slab memory.

Test Case detail:
Kernel version : 4.4
We are following stress-ng test.
https://github.com/ColinIanKing/stress-ng

1. dirdeep for creating directory upto maximum depth.
2. vm for memory allocation in backgroud.

Above 2 test cases are running simultaneously and it takes ~3-4 hours to 
reproduce this issue.

Pasting comment section for load_unaligned_zeropad():
/*
   * Load an unaligned word from kernel space.
   *
   * In the (very unlikely) case of the word being a page-crosser
   * and the next page not being mapped, take the exception and
   * return zeroes in the non-existing part.
   */

Wanted to confirm if this "very unlikely case" mentioned above is 
pointing to the same issue ?
Also let us know if this is a known issue and if some fix is already 
available. If not, please suggest how we can proceed further.

Appreciate your help. Thanks in advance!!

Regards,
Ankit Jain
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, 
Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a 
Linux Foundation Collaborative Project