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

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Date:   Thu, 30 Nov 2017 07:53:35 +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 Wed 29-11-17 13:41:59, Andrew Morton wrote:
> On Wed, 29 Nov 2017 17:04:46 +0100 Michal Hocko <mhocko@...nel.org> wrote:
> 
> > On Mon 27-11-17 12:33:41, Michal Hocko wrote:
> > > On Mon 27-11-17 19:09:24, JianKang Chen wrote:
> > > > From: Jiankang Chen <chenjiankang1@...wei.com>
> > > > 
> > > > __get_free_pages will return an virtual address, 
> > > > but it is not just 32-bit address, for example a 64-bit system. 
> > > > And this comment really confuse new bigenner of mm.
> > > 
> > > s@...enner@...inner@
> > > 
> > > Anyway, do we really need a bug on for this? Has this actually caught
> > > any wrong usage? VM_BUG_ON tends to be enabled these days AFAIK and
> > > panicking the kernel seems like an over-reaction. If there is a real
> > > risk then why don't we simply mask __GFP_HIGHMEM off when calling
> > > alloc_pages?
> > 
> > I meant this
> > ---
> > >From 000bb422fe07adbfa8cd8ed953b18f48647a45d6 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@...e.com>
> > Date: Wed, 29 Nov 2017 17:02:33 +0100
> > Subject: [PATCH] mm: drop VM_BUG_ON from __get_free_pages
> > 
> > There is no real reason to blow up just because the caller doesn't know
> > that __get_free_pages cannot return highmem pages. Simply fix that up
> > silently. Even if we have some confused users such a fixup will not be
> > harmful.
> 
> 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?

> 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.

-- 
Michal Hocko
SUSE Labs