Re: [PATCH 1/2] apparmor: Use a memory pool instead per-CPU caches

[John Johansen <john.johansen@canonical.com>]

On 4/30/19 7:47 AM, Sebastian Andrzej Siewior wrote:
> On 2019-04-28 16:56:59 [-0700], John Johansen wrote:
>> So digging into why the history of the per cpu buffers in apparmor.
>> We used to do buffer allocations via kmalloc and there were a few reasons
>> for the switch 
>>
>> * speed/lockless: speaks for it self, mediation is already slow enough
> 
> it is shared among all CPUs but it is a small/quick operation to
> add/return a buffer.
> 
I wouldn't exactly call taking a lock speedy. Getting an available buffer
or returning it is indeed quick. The allocation fall back not so much.


>> * some buffer allocations had to be done with GFP_ATOMIC, making them
>>   more likely to fail. Since we fail closed that means failure would
>>   block access. This actually became a serious problem in a couple
>>   places. Switching to per cpu buffers and blocking pre-empt was
>>   the solution.
> 
> GFP_KERNEL is allowed to use IO/SWAP and ATOMIC has emergency pools. The
> new approach won't return a NULL pointer, simply spin to either allocate
> new memory or get one which was just returned.
> 

yeah, I am not really a fan of a potential infinite loop trying to allocate
memory. It may be worth retrying once or twice but potentially infinitely
spinning on failed allocation really isn't acceptable.

>> * in heavy use cases we would see a lot of buffers being allocated
>>   and freed. Which resulted in locking slow downs and also buffer
>>   allocation failures. So having the buffers preallocated allowed us
>>   to bound this potential problem.
>>
>> This was all 6 years ago. Going to a mem pool certainly could help,
>> reduce the memory foot print, and would definitely help with
>> preempt/real time kernels.
>>
>> A big concern with this patchset is reverting back to GFP_KERNEL
>> for everything. We definitely were getting failures due to allocations
>> in atomic context. There have been lots of changes in the kernel over
>> the last six years so it possible these cases don't exist anymore. I
>> went through and built some kernels with this patchset and have run
>> through some testing without tripping that problem but I don't think
>> it has seen enough testing yet.
> 
> Do you want apply #1 now and #2 later? I audited the ATOMIC->KERNEL
> changes manually and I didn't see any atomic context. It looked like the
> only reason for ATOMIC was the preempt_disable() due to the memory pool.
> 

Indeed most if not all (I'd have to dig to be sure) the changes made in #2
were original done because of the move to the per cpu buffers and blocking
pre-emption.

The problem was with the allocation of the buffer needing to be GFP_ATOMIC
some times.