Recently I straced bash behavior in this dd zero pipe to read test,
in part of testing under vm.overcommit_memory=2 (OVERCOMMIT_NEVER mode):
# dd if=/dev/zero | read x
The bash sub shell is calling mremap to reallocate more and more memory
untill it finally failed -ENOMEM (I expect), or to be killed by system
OOM killer (which should not happen under OVERCOMMIT_NEVER mode);
But the mremap system call actually failed of -EFAULT, which is a
surprise to me, I think it's supposed to be -ENOMEM? then I wrote this
piece of C code testing confirmed it:
https://gist.github.com/crquan/326bde37e1ddda8effe5
$ ./remap
allocated one page @0x7f686bf71000, (PAGE_SIZE: 4096)
grabbed 7680512000 bytes of memory (1875125 pages) @ 00007f6690993000.
mremap failed Bad address (14).
The -EFAULT comes from the branch of security_vm_enough_memory_mm
failure, underlyingly it calls __vm_enough_memory which returns only
0 for success or -ENOMEM; So why vma_to_resize needs to return
-EFAULT in this case? this sounds like a mistake to me.
Some more digging into git history:
1) Before commit 119f657c7 in May 1 2005 (pre 2.6.12 days) it was
returning -ENOMEM for this failure;
2) but commit 119f657c7 changed it accidentally, to what ever is
preserved in local ret, which happened to be -EFAULT, in a previous assignment;
3) then in commit 54f5de709 code refactoring, it's explicitly returning
-EFAULT, should be wrong.
Signed-off-by: Derek Che <crquan@xxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
---
mm/mremap.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/mremap.c b/mm/mremap.c
index 57dadc0..5da81cb 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -375,7 +375,7 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
if (vma->vm_flags & VM_ACCOUNT) {
unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
if (security_vm_enough_memory_mm(mm, charged))
- goto Efault;
+ goto Enomem;
*p = charged;
}
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