Re: [PATCHv5 07/28] thp, mlock: do not allow huge pages in mlocked area

[Vlastimil Babka]

On 04/23/2015 11:03 PM, Kirill A. Shutemov wrote:
> With new refcounting THP can belong to several VMAs. This makes tricky
> to track THP pages, when they partially mlocked. It can lead to leaking
> mlocked pages to non-VM_LOCKED vmas and other problems.
>
> With this patch we will split all pages on mlock and avoid
> fault-in/collapse new THP in VM_LOCKED vmas.
>
> I've tried alternative approach: do not mark THP pages mlocked and keep
> them on normal LRUs. This way vmscan could try to split huge pages on
> memory pressure and free up subpages which doesn't belong to VM_LOCKED
> vmas.  But this is user-visible change: we screw up Mlocked accouting
> reported in meminfo, so I had to leave this approach aside.
>
> We can bring something better later, but this should be good enough for
> now.

I can imagine people won't be happy about losing benefits of THP's when 
they mlock().
How difficult would it be to support mlocked THP pages without splitting 
until something actually tries to do a partial (un)mapping, and only 
then do the split? That will support the most common case, no?

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx>
> ---
>   mm/gup.c         |  2 ++
>   mm/huge_memory.c |  5 ++++-
>   mm/memory.c      |  3 ++-
>   mm/mlock.c       | 51 +++++++++++++++++++--------------------------------
>   4 files changed, 27 insertions(+), 34 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index eaeeae15006b..7334eb24f414 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -882,6 +882,8 @@ long populate_vma_page_range(struct vm_area_struct *vma,
>   	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
>
>   	gup_flags = FOLL_TOUCH | FOLL_POPULATE;
> +	if (vma->vm_flags & VM_LOCKED)
> +		gup_flags |= FOLL_SPLIT;
>   	/*
>   	 * We want to touch writable mappings with a write fault in order
>   	 * to break COW, except for shared mappings because these don't COW
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index fd8af5b9917f..fa3d4f78b716 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -796,6 +796,8 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
>
>   	if (haddr < vma->vm_start || haddr + HPAGE_PMD_SIZE > vma->vm_end)
>   		return VM_FAULT_FALLBACK;
> +	if (vma->vm_flags & VM_LOCKED)
> +		return VM_FAULT_FALLBACK;
>   	if (unlikely(anon_vma_prepare(vma)))
>   		return VM_FAULT_OOM;
>   	if (unlikely(khugepaged_enter(vma, vma->vm_flags)))
> @@ -2467,7 +2469,8 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
>   	if ((!(vma->vm_flags & VM_HUGEPAGE) && !khugepaged_always()) ||
>   	    (vma->vm_flags & VM_NOHUGEPAGE))
>   		return false;
> -
> +	if (vma->vm_flags & VM_LOCKED)
> +		return false;
>   	if (!vma->anon_vma || vma->vm_ops)
>   		return false;
>   	if (is_vma_temporary_stack(vma))
> diff --git a/mm/memory.c b/mm/memory.c
> index 559c6651d6b6..8bbd3f88544b 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2156,7 +2156,8 @@ static int wp_page_copy(struct mm_struct *mm, struct vm_area_struct *vma,
>
>   	pte_unmap_unlock(page_table, ptl);
>   	mmu_notifier_invalidate_range_end(mm, mmun_start, mmun_end);
> -	if (old_page) {
> +	/* THP pages are never mlocked */
> +	if (old_page && !PageTransCompound(old_page)) {
>   		/*
>   		 * Don't let another task, with possibly unlocked vma,
>   		 * keep the mlocked page.
> diff --git a/mm/mlock.c b/mm/mlock.c
> index 6fd2cf15e868..76cde3967483 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -443,39 +443,26 @@ void munlock_vma_pages_range(struct vm_area_struct *vma,
>   		page = follow_page_mask(vma, start, FOLL_GET | FOLL_DUMP,
>   				&page_mask);
>
> -		if (page && !IS_ERR(page)) {
> -			if (PageTransHuge(page)) {
> -				lock_page(page);
> -				/*
> -				 * Any THP page found by follow_page_mask() may
> -				 * have gotten split before reaching
> -				 * munlock_vma_page(), so we need to recompute
> -				 * the page_mask here.
> -				 */
> -				page_mask = munlock_vma_page(page);
> -				unlock_page(page);
> -				put_page(page); /* follow_page_mask() */
> -			} else {
> -				/*
> -				 * Non-huge pages are handled in batches via
> -				 * pagevec. The pin from follow_page_mask()
> -				 * prevents them from collapsing by THP.
> -				 */
> -				pagevec_add(&pvec, page);
> -				zone = page_zone(page);
> -				zoneid = page_zone_id(page);
> +		if (page && !IS_ERR(page) && !PageTransCompound(page)) {
> +			/*
> +			 * Non-huge pages are handled in batches via
> +			 * pagevec. The pin from follow_page_mask()
> +			 * prevents them from collapsing by THP.
> +			 */
> +			pagevec_add(&pvec, page);
> +			zone = page_zone(page);
> +			zoneid = page_zone_id(page);
>
> -				/*
> -				 * Try to fill the rest of pagevec using fast
> -				 * pte walk. This will also update start to
> -				 * the next page to process. Then munlock the
> -				 * pagevec.
> -				 */
> -				start = __munlock_pagevec_fill(&pvec, vma,
> -						zoneid, start, end);
> -				__munlock_pagevec(&pvec, zone);
> -				goto next;
> -			}
> +			/*
> +			 * Try to fill the rest of pagevec using fast
> +			 * pte walk. This will also update start to
> +			 * the next page to process. Then munlock the
> +			 * pagevec.
> +			 */
> +			start = __munlock_pagevec_fill(&pvec, vma,
> +					zoneid, start, end);
> +			__munlock_pagevec(&pvec, zone);
> +			goto next;
>   		}
>   		/* It's a bug to munlock in the middle of a THP page */
>   		VM_BUG_ON((start >> PAGE_SHIFT) & page_mask);

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