[PATCH 3.2 85/94] mm: hugetlb: fix copy_hugetlb_page_range()

[Ben Hutchings]

3.2.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>

commit 0253d634e0803a8376a0d88efee0bf523d8673f9 upstream.

Commit 4a705fef9862 ("hugetlb: fix copy_hugetlb_page_range() to handle
migration/hwpoisoned entry") changed the order of
huge_ptep_set_wrprotect() and huge_ptep_get(), which leads to breakage
in some workloads like hugepage-backed heap allocation via libhugetlbfs.
This patch fixes it.

The test program for the problem is shown below:

  $ cat heap.c
  #include <unistd.h>
  #include <stdlib.h>
  #include <string.h>

  #define HPS 0x200000

  int main() {
  	int i;
  	char *p = malloc(HPS);
  	memset(p, '1', HPS);
  	for (i = 0; i < 5; i++) {
  		if (!fork()) {
  			memset(p, '2', HPS);
  			p = malloc(HPS);
  			memset(p, '3', HPS);
  			free(p);
  			return 0;
  		}
  	}
  	sleep(1);
  	free(p);
  	return 0;
  }

  $ export HUGETLB_MORECORE=yes ; export HUGETLB_NO_PREFAULT= ; hugectl --heap ./heap

Fixes 4a705fef9862 ("hugetlb: fix copy_hugetlb_page_range() to handle
migration/hwpoisoned entry"), so is applicable to -stable kernels which
include it.

Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Reported-by: Guillaume Morin <guillaume@xxxxxxxxxxx>
Suggested-by: Guillaume Morin <guillaume@xxxxxxxxxxx>
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/hugetlb.c | 1 +
 1 file changed, 1 insertion(+)

--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2344,6 +2344,7 @@ int copy_hugetlb_page_range(struct mm_st
 		} else {
 			if (cow)
 				huge_ptep_set_wrprotect(src, addr, src_pte);
+			entry = huge_ptep_get(src_pte);
 			ptepage = pte_page(entry);
 			get_page(ptepage);
 			page_dup_rmap(ptepage);

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