Re: [PATCH 5/7] khugepaged: Allow to collapse PTE-mapped compound pages

Yang Shi
2020-03-27T20:46:11Z

On Fri, Mar 27, 2020 at 10:06 AM Kirill A. Shutemov
<kirill@shutemov.name> wrote:
>
> We can collapse PTE-mapped compound pages. We only need to avoid
> handling them more than once: lock/unlock page only once if it's present
> in the PMD range multiple times as it handled on compound level. The
> same goes for LRU isolation and putpack.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> mm/khugepaged.c | 41 +++++++++++++++++++++++++++++++----------
> 1 file changed, 31 insertions(+), 10 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b47edfe57f7b..c8c2c463095c 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -515,6 +515,17 @@ void __khugepaged_exit(struct mm_struct *mm)
>
> static void release_pte_page(struct page *page)
> {
> + /*
> + * We need to unlock and put compound page on LRU only once.
> + * The rest of the pages have to be locked and not on LRU here.
> + */
> + VM_BUG_ON_PAGE(!PageCompound(page) &&
> + (!PageLocked(page) && PageLRU(page)), page);
> +
> + if (!PageLocked(page))
> + return;
> +
> + page = compound_head(page);
> dec_node_page_state(page, NR_ISOLATED_ANON + page_is_file_cache(page));
> unlock_page(page);
> putback_lru_page(page);

BTW, wouldn't this unlock the whole THP and put it back to LRU? Then
we may copy the following PTE mapped pages with page unlocked and on
LRU. I don't see critical problem, just the pages might be on and off
LRU by others, i.e. vmscan, compaction, migration, etc. But no one
could take the page away since try_to_unmap() would fail, but not very
productive.

> @@ -537,6 +548,7 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> pte_t *_pte;
> int none_or_zero = 0, result = 0, referenced = 0;
> bool writable = false;
> + LIST_HEAD(compound_pagelist);
>
> for (_pte = pte; _pte < pte+HPAGE_PMD_NR;
> _pte++, address += PAGE_SIZE) {
> @@ -561,13 +573,23 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> goto out;
> }
>
> - /* TODO: teach khugepaged to collapse THP mapped with pte */
> + VM_BUG_ON_PAGE(!PageAnon(page), page);
> +
> if (PageCompound(page)) {
> - result = SCAN_PAGE_COMPOUND;
> - goto out;
> - }
> + struct page *p;
> + page = compound_head(page);
>
> - VM_BUG_ON_PAGE(!PageAnon(page), page);
> + /*
> + * Check if we have dealt with the compount page
> + * already
> + */
> + list_for_each_entry(p, &compound_pagelist, lru) {
> + if (page == p)
> + break;
> + }
> + if (page == p)
> + continue;
> + }
>
> /*
> * We can do it before isolate_lru_page because the
> @@ -640,6 +662,9 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> page_is_young(page) || PageReferenced(page) ||
> mmu_notifier_test_young(vma->vm_mm, address))
> referenced++;
> +
> + if (PageCompound(page))
> + list_add_tail(&page->lru, &compound_pagelist);
> }
> if (likely(writable)) {
> if (likely(referenced)) {
> @@ -1185,11 +1210,7 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> goto out_unmap;
> }
>
> - /* TODO: teach khugepaged to collapse THP mapped with pte */
> - if (PageCompound(page)) {
> - result = SCAN_PAGE_COMPOUND;
> - goto out_unmap;
> - }
> + page = compound_head(page);
>
> /*
> * Record which node the original page is from and save this
> --
> 2.26.0
>
>