linux-kernel - Re: [PATCH resend] mm/page_alloc: fix comment is __get_free

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Message-ID: <20171214140608.GQ16951@dhcp22.suse.cz>
Date:   Thu, 14 Dec 2017 15:06:08 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     JianKang Chen <chenjiankang1@...wei.com>,
        mgorman@...hsingularity.net, hannes@...xchg.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        xieyisheng1@...wei.com, guohanjun@...wei.com,
        wangkefeng.wang@...wei.com
Subject: Re: [PATCH resend] mm/page_alloc: fix comment is __get_free_pages

On Fri 01-12-17 12:18:45, Michal Hocko wrote:
> On Fri 01-12-17 08:24:14, Michal Hocko wrote:
> > On Thu 30-11-17 13:17:06, Andrew Morton wrote:
> > > On Thu, 30 Nov 2017 07:53:35 +0100 Michal Hocko <mhocko@...nel.org> 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.
> 
> let me be more specific. They will blow up and leak if the returned
> address is not checked. If it is then we just leak. None of that sounds
> good to me.

So do we care and I will resend the patch in that case or I just drop
this from my patch queue?

-- 
Michal Hocko
SUSE Labs