Re: [PATCH] mremap should return -ENOMEM when __vm_enough_memory fail

[David Rientjes <rientjes@google.com>]

On Fri, 13 Mar 2015, Derek wrote:

> 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
> 
> 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?
> it 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@ymail.com>

Acked-by: David Rientjes <rientjes@google.com>

vma_to_resize() could certainly be cleaned up to just return ERR_PTR() and 
avoiding the "goto"s since there is no other cleanup needed as suggested 
by Kirill if you have time for a cleanup patch on top of this.

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