Re: Page fault while link_path_walk for path_len > 4060 bytes

[ankijain@codeaurora.org]

On 2017-08-07 11:52, ankijain@codeaurora.org wrote:
> 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