Re: [PATCH v2] mm, page_alloc: disallow __GFP_COMP in alloc_pages_exact()

[Michal Hocko <mhocko@suse.com>]

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