Re: suspicious __GFP_NOMEMALLOC in selinux

[Michal Hocko <mhocko@kernel.org>]

On Thu 03-08-17 19:02:57, Tetsuo Handa wrote:
> 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@kernel.org> 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 ?

This would require an additional justification.

> And why not also __GFP_NOMEMALLOC ?

What would be the purpose of __GFP_NOMEMALLOC? In other words which
context would set PF_NOMEMALLOC so that the flag would override it?

-- 
Michal Hocko
SUSE Labs

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