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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Jan Stancek <jstancek@redhat.com> writes:


> 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.

ok, we miss the _PAGE_PTE in new_pte there. 

	new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
				&& (vma->vm_flags & VM_SHARED)));
	set_huge_pte_at(mm, address, ptep, new_pte);

	hugetlb_count_add(pages_per_huge_page(h), mm);
	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
		/* Optimization, do the COW without a second fault */
		ret = hugetlb_cow(mm, vma, address, ptep, new_pte, page, ptl);
	}

IMHO that new_pte usage is wrong, because we don't consider flags that
can possibly be added by set_huge_pte_at there. For pp64 we add _PAGE_PTE 

>
> 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
>

But I guess we still have issue with respecting reservation here.

I will look at _PAGE_PTE and see what best we can do w.r.t hugetlb.

-aneesh

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