linux-kernel - [PATCH] mm: drop VM_BUG_ON from __get_free

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Date:   Fri, 22 Jun 2018 18:28:41 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     JianKang Chen <chenjiankang1@...wei.com>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, xieyisheng1@...wei.com,
        guohanjun@...wei.com, wangkefeng.wang@...wei.com,
        Michal Hocko <mhocko@...e.com>
Subject: [PATCH] mm: drop VM_BUG_ON from __get_free_pages

From: Michal Hocko <mhocko@...e.com>

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>
---
Hi Andrew,
previously posted [1] but it fell through cracks. Can we merge it now?

[1] http://lkml.kernel.org/r/20171129160446.jluzpv3n6mjc3fwv@dhcp22.suse.cz

 mm/page_alloc.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1521100f1e63..5f56f662a52d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4402,18 +4402,14 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order, int preferred_nid,
 EXPORT_SYMBOL(__alloc_pages_nodemask);
 
 /*
- * Common helper functions.
+ * Common helper functions. Never use with __GFP_HIGHMEM because the returned
+ * address cannot represent highmem pages. Use alloc_pages and then kmap if
+ * you need to access high mem.
  */
 unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
 {
 	struct page *page;
 
-	/*
-	 * __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);
 	if (!page)
 		return 0;
-- 
2.17.1