Re: [PATCH] mm: stop leaking PageTables

From: Aneesh Kumar K.V
Date: Sun Jan 08 2017 - 02:00:01 EST


Hugh Dickins <hughd@xxxxxxxxxx> writes:

> 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.

That change is part of -mm tree.

https://lkml.kernel.org/r/20161212163428.6780-1-aneesh.kumar@xxxxxxxxxxxxxxxxxx

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


Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>

> ---
>
> 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,