APPLIED/cmnt: [X][X/linux-kvm][SRU][PATCH 0/1] mm: mlock: avoid increase mm->locked_vm on mlock() when already mlock2(, MLOCK_ONFAULT)

[Khaled Elmously]

Applied to xenial/master-next only

On 2018-10-23 16:44:51 , Po-Hsu Lin wrote:
> BugLink: https://bugs.launchpad.net/bugs/1793451
> 
> == Justification ==
> When one vma was with flag VM_LOCKED|VM_LOCKONFAULT (by invoking
> mlock2(,MLOCK_ONFAULT)), it can again be populated with mlock() with
> VM_LOCKED flag only.
> 
> There is a hole in mlock_fixup() which increase mm->locked_vm twice even
> the two operations are on the same vma and both with VM_LOCKED flags.
> 
> The issue can be reproduced by following code:
> 
>   mlock2(p, 1024 * 64, MLOCK_ONFAULT); //VM_LOCKED|VM_LOCKONFAULT
>   mlock(p, 1024 * 64); //VM_LOCKED
> 
> Then check the increase VmLck field in /proc/pid/status(to 128k).
> 
> When vma is set with different vm_flags, and the new vm_flags is with
> VM_LOCKED, it is not necessarily be a "new locked" vma.
> 
> There is a dedicated reproducer, the "mlock203" test in ubuntu_ltp_syscalls, you can see the failure for all the Ubuntu 4.4 kernel:
> 
>  <<<test_start>>>
>  tag=mlock203 stime=1537369891
>  cmdline="mlock203"
>  contacts=""
>  analysis=exit
>  <<<test_output>>>
>  tst_test.c:1063: INFO: Timeout per run is 0h 05m 00s
>  mlock203.c:63: FAIL: Locking one memory again increased VmLck
> 
>  Summary:
>  passed 0
>  failed 1
>  skipped 0
>  warnings 0
> 
> == Fix ==
> b155b4fd (mm: mlock: avoid increase mm->locked_vm on mlock() when already mlock2(,MLOCK_ONFAULT))
> 
> A test kernel for Xenial / Xenial-KVM could be found here:
> http://people.canonical.com/~phlin/kernel/lp-1793451-mlock203/
> 
> == Regression Potential ==
> Low, this patch prevents mm->locked_vm from increment just by adding an extra check to see if the old vm_flags is already VM_LOCKED.
> 
> == Test Case ==
> Run the mlock203 test in ubuntu_ltp_syscalls test suite. And it will pass with the patched kernel.
> 
> Simon Guo (1):
>   mm: mlock: avoid increase mm->locked_vm on mlock() when already
>     mlock2(,MLOCK_ONFAULT)
> 
>  mm/mlock.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> -- 
> 2.7.4
> 
> 
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