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

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Date:   Wed, 29 Nov 2017 17:04:46 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     JianKang Chen <chenjiankang1@...wei.com>
Cc:     akpm@...ux-foundation.org, 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 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.

Signed-off-by: Michal Hocko <mhocko@...e.com>
---
 mm/page_alloc.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0d518e9b2ee8..3dd960ea8c13 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4284,9 +4284,7 @@ unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
 	 * __get_free_pages() returns a virtual address, which cannot represent
 	 * a highmem page
 	 */
-	VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
-
-	page = alloc_pages(gfp_mask, order);
+	page = alloc_pages(gfp_mask & ~__GFP_HIGHMEM, order);
 	if (!page)
 		return 0;
 	return (unsigned long) page_address(page);
-- 
2.15.0

-- 
Michal Hocko
SUSE Labs