On Thu 14-03-19 14:15:38, Takashi Iwai wrote: > On Thu, 14 Mar 2019 13:09:39 +0100, > Michal Hocko wrote: > > > > On Thu 14-03-19 12:56:43, Takashi Iwai wrote: > > > On Thu, 14 Mar 2019 12:36:26 +0100, > > > Michal Hocko wrote: > > > > > > > > On Thu 14-03-19 11:30:03, Vlastimil Babka wrote: [...] > > > > > I initially went with 2 as well, as you can see from v1 :) but then I looked at > > > > > the commit [2] mentioned in [1] and I think ALSA legitimaly uses __GFP_COMP so > > > > > that the pages are then mapped to userspace. Breaking that didn't seem good. > > > > > > > > It used the flag legitimately before because they were allocating > > > > compound pages but now they don't so this is just a conversion bug. > > > > > > We still use __GFP_COMP for allocation of the sound buffers that are > > > also mmapped to user-space. The mentioned commit above [2] was > > > reverted later. > > > > Yes, I understand that part. __GFP_COMP makes sense on a comound page. > > But if you are using alloc_pages_exact then the flag doesn't make sense > > because split out should already do what you want. Unless I am missing > > something. > > The __GFP_COMP was taken as a sort of workaround for the problem wrt > mmap I already forgot. If it can be eliminated, it's all good. Without __GFP_COMP you would get tail pages which are not setup properly AFAIU. With alloc_pages_exact you should get an "array" of head pages which are properly reference counted. But I might misunderstood the original problem which __GFP_COMP tried to solve. -- Michal Hocko SUSE Labs
