Re: Page fault while link_path_walk for path_len > 4060 bytes

[ankijain@codeaurora.org]

Hi linux-fsdevel

could you please look at this issue or suggest how we can proceed 
further.

Thanks,

On 2017-08-07 17:48, ankijain@codeaurora.org wrote:
> 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