Re: [PATCH resend] mm/page_alloc: fix comment is __get_free_pages

[Michal Hocko]

On Thu 30-11-17 13:17:06, Andrew Morton wrote:
> On Thu, 30 Nov 2017 07:53:35 +0100 Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> 
> > > mm...  So we have a caller which hopes to be getting highmem pages but
> > > isn't.  Caller then proceeds to pointlessly kmap the page and wonders
> > > why it isn't getting as much memory as it would like on 32-bit systems,
> > > etc.
> > 
> > How he can kmap the page when he gets a _virtual_ address?
> 
> doh.
> 
> > > I do think we should help ferret out such bogosity.  A WARN_ON_ONCE
> > > would suffice.
> > 
> > This function has always been about lowmem pages. I seriously doubt we
> > have anybody confused and asking for a highmem page in the kernel. I
> > haven't checked that but it would already blow up as VM_BUG_ON tends to
> > be enabled on many setups.
> 
> OK.  But silently accepting __GFP_HIGHMEM is a bit weird - callers
> shouldn't be doing that in the first place.

Yes, they shouldn't be.

> I wonder what happens if we just remove the WARN_ON and pass any
> __GFP_HIGHMEM straight through.  The caller gets a weird address from
> page_to_virt(highmem page) and usually goes splat?  Good enough
> treatment for something which never happens anyway?

page_address will return NULL so they will blow up and leak the freshly
allocated memory. I do not think this is really desirable. We _could_
handle this case but I am not really sure this is a win. A silent fixup
sounds like the most simply protection.

-- 
Michal Hocko
SUSE Labs