Re: [bug/regression] libhugetlbfs testsuite failures and OOMs eventually kill my system

[Jan Stancek]

----- Original Message -----
> From: "Mike Kravetz" <mike.kravetz@xxxxxxxxxx>
> To: "Jan Stancek" <jstancek@xxxxxxxxxx>, linux-mm@xxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Cc: "hillf zj" <hillf.zj@xxxxxxxxxxxxxxx>, "dave hansen" <dave.hansen@xxxxxxxxxxxxxxx>, "kirill shutemov"
> <kirill.shutemov@xxxxxxxxxxxxxxx>, mhocko@xxxxxxx, n-horiguchi@xxxxxxxxxxxxx, "aneesh kumar"
> <aneesh.kumar@xxxxxxxxxxxxxxxxxx>, "iamjoonsoo kim" <iamjoonsoo.kim@xxxxxxx>
> Sent: Saturday, 15 October, 2016 1:57:31 AM
> Subject: Re: [bug/regression] libhugetlbfs testsuite failures and OOMs eventually kill my system
> 
> 
> It is pretty consistent that we leak a reserve page every time this
> test is run.
> 
> The interesting thing is that corrupt-by-cow-opt is a very simple
> test case.  commit 67961f9db8c4 potentially changes the return value
> of the functions vma_has_reserves() and vma_needs/commit_reservation()
> for the owner (HPAGE_RESV_OWNER) of private mappings.  running the
> test with and without the commit results in the same return values for
> these routines on x86.  And, no leaked reserve pages.
> 
> Is it possible to revert this commit and run the libhugetlbs tests
> (func and stress) again while monitoring the counts in /sys?  The
> counts should go to zero after cleanup as you describe above.  I just
> want to make sure that this commit is causing all the problems you
> are seeing.  If it is, then we can consider reverting and I can try
> to think of another way to address the original issue.
> 
> Thanks for your efforts on this.  I can not reproduce on x86 or sparc
> and do not see any similar symptoms on these architectures.
> 
> --
> Mike Kravetz
> 

Hi Mike,

Revert of 67961f9db8c4 helps, I let whole suite run for 100 iterations,
there were no issues.

I cut down reproducer and removed last mmap/write/munmap as that is enough
to reproduce the problem. Then I started introducing some traces into kernel
and noticed that on ppc I get 3 faults, while on x86 I get only 2.

Interesting is the 2nd fault, that is first write after mapping as PRIVATE.
Following condition fails on ppc first time:
    if (likely(ptep && pte_same(huge_ptep_get(ptep), pte))) {
but it's immediately followed by fault that looks identical
and in that one it evaluates as true.

Same with alloc_huge_page(), on x86_64 it's called twice, on ppc three times.
In 2nd call vma_needs_reservation() returns 0, in 3rd it returns 1.

---- ppc -> 2nd and 3rd fault ---
mmap(MAP_PRIVATE)
hugetlb_fault address: 3effff000000, flags: 55
hugetlb_cow old_page: f0000000010fc000
alloc_huge_page ret: f000000001100000
hugetlb_cow ptep: c000000455b27cf8, pte_same: 0
free_huge_page page: f000000001100000, restore_reserve: 1
hugetlb_fault address: 3effff000000, flags: 55
hugetlb_cow old_page: f0000000010fc000
alloc_huge_page ret: f000000001100000
hugetlb_cow ptep: c000000455b27cf8, pte_same: 1

--- x86_64 -> 2nd fault ---
mmap(MAP_PRIVATE)
hugetlb_fault address: 7f71a4200000, flags: 55
hugetlb_cow address 0x7f71a4200000, old_page: ffffea0008d20000
alloc_huge_page ret: ffffea0008d38000
hugetlb_cow ptep: ffff8802314c7908, pte_same: 1

Regards,
Jan