Re: suspicious __GFP_NOMEMALLOC in selinux

[Tetsuo Handa]

On 2017/08/03 17:11, Michal Hocko wrote:
> [CC Mel]
> 
> On Wed 02-08-17 17:45:56, Paul Moore wrote:
>> On Wed, Aug 2, 2017 at 6:50 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>>> Hi,
>>> while doing something completely unrelated to selinux I've noticed a
>>> really strange __GFP_NOMEMALLOC usage pattern in selinux, especially
>>> GFP_ATOMIC | __GFP_NOMEMALLOC doesn't make much sense to me. GFP_ATOMIC
>>> on its own allows to access memory reserves while the later flag tells
>>> we cannot use memory reserves at all. The primary usecase for
>>> __GFP_NOMEMALLOC is to override a global PF_MEMALLOC should there be a
>>> need.
>>>
>>> It all leads to fa1aa143ac4a ("selinux: extended permissions for
>>> ioctls") which doesn't explain this aspect so let me ask. Why is the
>>> flag used at all? Moreover shouldn't GFP_ATOMIC be actually GFP_NOWAIT.
>>> What makes this path important to access memory reserves?
>>
>> [NOTE: added the SELinux list to the CC line, please include that list
>> when asking SELinux questions]
> 
> Sorry about that. Will keep it in mind for next posts
>  
>> The GFP_ATOMIC|__GFP_NOMEMALLOC use in SELinux appears to be limited
>> to security/selinux/avc.c, and digging a bit, I'm guessing commit
>> fa1aa143ac4a copied the combination from 6290c2c43973 ("selinux: tag
>> avc cache alloc as non-critical") and the avc_alloc_node() function.
> 
> Thanks for the pointer. That makes much more sense now. Back in 2012 we
> really didn't have a good way to distinguish non sleeping and atomic
> with reserves allocations.
>  
>> I can't say that I'm an expert at the vm subsystem and the variety of
>> different GFP_* flags, but your suggestion of moving to GFP_NOWAIT in
>> security/selinux/avc.c seems reasonable and in keeping with the idea
>> behind commit 6290c2c43973.
> 
> What do you think about the following? I haven't tested it but it should
> be rather straightforward.

Why not at least __GFP_NOWARN ? And why not also __GFP_NOMEMALLOC ?
http://lkml.kernel.org/r/201706302210.GCA05089.MFFOtQVJSOLHOF@xxxxxxxxxxxxxxxxxxx