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@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 ? 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>