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    Subject[PATCH] mm: stop leaking PageTables
    4.10-rc loadtest (even on x86, even without THPCache) fails with
    "fork: Cannot allocate memory" or some such; and /proc/meminfo
    shows PageTables growing.

    rc1 removed the freeing of an unused preallocated pagetable after
    do_fault_around() has called map_pages(): which is usually a good
    optimization, so that the followup doesn't have to reallocate one;
    but it's not sufficient to shift the freeing into alloc_set_pte(),
    since there are failure cases (most commonly VM_FAULT_RETRY) which
    never reach finish_fault().

    Check and free it at the outer level in do_fault(), then we don't
    need to worry in alloc_set_pte(), and can restore that to how it was
    (I cannot find any reason to pte_free() under lock as it was doing).

    And fix a separate pagetable leak, or crash, introduced by the same
    change, that could only show up on some ppc64: why does do_set_pmd()'s
    failure case attempt to withdraw a pagetable when it never deposited
    one, at the same time overwriting (so leaking) the vmf->prealloc_pte?
    Residue of an earlier implementation, perhaps? Delete it.

    Fixes: 953c66c2b22a ("mm: THP page cache support for ppc64")
    Signed-off-by: Hugh Dickins <hughd@google.com>
    ---

    mm/memory.c | 47 ++++++++++++++++++++---------------------------
    1 file changed, 20 insertions(+), 27 deletions(-)

    --- 4.10-rc2/mm/memory.c 2016-12-25 18:40:50.830453384 -0800
    +++ linux/mm/memory.c 2017-01-07 13:34:29.373381551 -0800
    @@ -3008,13 +3008,6 @@ static int do_set_pmd(struct vm_fault *v
    ret = 0;
    count_vm_event(THP_FILE_MAPPED);
    out:
    - /*
    - * If we are going to fallback to pte mapping, do a
    - * withdraw with pmd lock held.
    - */
    - if (arch_needs_pgtable_deposit() && ret == VM_FAULT_FALLBACK)
    - vmf->prealloc_pte = pgtable_trans_huge_withdraw(vma->vm_mm,
    - vmf->pmd);
    spin_unlock(vmf->ptl);
    return ret;
    }
    @@ -3055,20 +3048,18 @@ int alloc_set_pte(struct vm_fault *vmf,

    ret = do_set_pmd(vmf, page);
    if (ret != VM_FAULT_FALLBACK)
    - goto fault_handled;
    + return ret;
    }

    if (!vmf->pte) {
    ret = pte_alloc_one_map(vmf);
    if (ret)
    - goto fault_handled;
    + return ret;
    }

    /* Re-check under ptl */
    - if (unlikely(!pte_none(*vmf->pte))) {
    - ret = VM_FAULT_NOPAGE;
    - goto fault_handled;
    - }
    + if (unlikely(!pte_none(*vmf->pte)))
    + return VM_FAULT_NOPAGE;

    flush_icache_page(vma, page);
    entry = mk_pte(page, vma->vm_page_prot);
    @@ -3088,15 +3079,8 @@ int alloc_set_pte(struct vm_fault *vmf,

    /* no need to invalidate: a not-present page won't be cached */
    update_mmu_cache(vma, vmf->address, vmf->pte);
    - ret = 0;

    -fault_handled:
    - /* preallocated pagetable is unused: free it */
    - if (vmf->prealloc_pte) {
    - pte_free(vmf->vma->vm_mm, vmf->prealloc_pte);
    - vmf->prealloc_pte = 0;
    - }
    - return ret;
    + return 0;
    }


    @@ -3360,15 +3344,24 @@ static int do_shared_fault(struct vm_fau
    static int do_fault(struct vm_fault *vmf)
    {
    struct vm_area_struct *vma = vmf->vma;
    + int ret;

    /* The VMA was not fully populated on mmap() or missing VM_DONTEXPAND */
    if (!vma->vm_ops->fault)
    - return VM_FAULT_SIGBUS;
    - if (!(vmf->flags & FAULT_FLAG_WRITE))
    - return do_read_fault(vmf);
    - if (!(vma->vm_flags & VM_SHARED))
    - return do_cow_fault(vmf);
    - return do_shared_fault(vmf);
    + ret = VM_FAULT_SIGBUS;
    + else if (!(vmf->flags & FAULT_FLAG_WRITE))
    + ret = do_read_fault(vmf);
    + else if (!(vma->vm_flags & VM_SHARED))
    + ret = do_cow_fault(vmf);
    + else
    + ret = do_shared_fault(vmf);
    +
    + /* preallocated pagetable is unused: free it */
    + if (vmf->prealloc_pte) {
    + pte_free(vma->vm_mm, vmf->prealloc_pte);
    + vmf->prealloc_pte = 0;
    + }
    + return ret;
    }

    static int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
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