[PATCH] mm: hugetlb: fix copy_hugetlb_page_range() (Re: [BUG] new copy_hugetlb_page_range() causing crashes)

[Naoya Horiguchi]

# CCed Andrew, and linux-mm 

On Thu, Jul 17, 2014 at 11:59:36PM +0200, Guillaume Morin wrote:
> On 17 Jul 17:33, Naoya Horiguchi wrote:
...
> > And it seems that this also happens on v3.16-rc5.
> > So it might be an upstream bug, not a stable-specific matter.
> 
> That's my understanding as well. I just reported it for 3.4 and 3.14
> since these were the kernels I could easily try my original test with.

OK. I've checked the fix you suggested below on mainline, and
it passed our test program.

> > It looks strange to me that the problem is gone by removing the commit
> > 4a705fef98 (although I confirmed it is,) because the kernel's behavior
> > shouldn't change unless (is_hugetlb_entry_migration(entry) ||
> > is_hugetlb_entry_hwpoisoned(entry)) is true. And I checked with systemtap
> > that both these check returned false in the above test program.
> > So I'm wondering why the commit makes difference for this test program.
> 
> I don't know why I am just thinking about that now.  Isn't this the fact
> that your patch moved the huge_ptep_get() before
> huge_ptep_set_wrprotect() in the pte_present() cow case?

Ah, right. I was really blind :(

> 
> Actually, I've just tried to re-add the huge_ptep_get call for that
> case and it's fixing the problem for me...
> 
> Hmm, want a patch?

Thanks, but it's just a oneliner, so I wrote the one.

Naoya Horiguchi
---
From: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Date: Thu, 17 Jul 2014 18:11:22 -0400
Subject: [PATCH] mm: hugetlb: fix copy_hugetlb_page_range()

commit 4a705fef98 ("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 break some workload 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

Reported-by: Guillaume Morin <guillaume@xxxxxxxxxxx>
Suggested-by: Guillaume Morin <guillaume@xxxxxxxxxxx>
Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 mm/hugetlb.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index a8d4155eb019..7263c770e9b3 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2597,6 +2597,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
 		} 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);
-- 
1.9.3

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