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SubjectRe: [PATCHv3, RESEND 2/8] khugepaged: Do not stop collapse if less than half PTEs are referenced
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On 4/13/20 5:52 AM, Kirill A. Shutemov wrote:
> __collapse_huge_page_swapin() checks the number of referenced PTE to
> decide if the memory range is hot enough to justify swapin.
>
> We have few problems with the approach:
>
> - It is way too late: we can do the check much earlier and safe time.
> khugepaged_scan_pmd() already knows if we have any pages to swap in
> and number of referenced page.
>
> - It stops collapse altogether if there's not enough referenced pages,
> not only swappingin.
>
> Fix it by making the right check early. We also can avoid additional
> page table scanning if khugepaged_scan_pmd() haven't found any swap
> entries.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: 0db501f7a34c ("mm, thp: convert from optimistic swapin collapsing to conservative")
> ---
> mm/khugepaged.c | 25 ++++++++++---------------
> 1 file changed, 10 insertions(+), 15 deletions(-)

Acked-by: Yang Shi <yang.shi@linux.alibaba.com>

Just a nit below.

>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 99bab7e4d05b..5968ec5ddd6b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -902,11 +902,6 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> .pgoff = linear_page_index(vma, address),
> };
>
> - /* we only decide to swapin, if there is enough young ptes */
> - if (referenced < HPAGE_PMD_NR/2) {
> - trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> - return false;
> - }
> vmf.pte = pte_offset_map(pmd, address);
> for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
> vmf.pte++, vmf.address += PAGE_SIZE) {
> @@ -946,7 +941,7 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> static void collapse_huge_page(struct mm_struct *mm,
> unsigned long address,
> struct page **hpage,
> - int node, int referenced)
> + int node, int referenced, int unmapped)
> {
> pmd_t *pmd, _pmd;
> pte_t *pte;
> @@ -1003,7 +998,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> * If it fails, we release mmap_sem and jump out_nolock.
> * Continuing to collapse causes inconsistency.
> */
> - if (!__collapse_huge_page_swapin(mm, vma, address, pmd, referenced)) {
> + if (unmapped && !__collapse_huge_page_swapin(mm, vma, address,
> + pmd, referenced)) {
> mem_cgroup_cancel_charge(new_page, memcg, true);
> up_read(&mm->mmap_sem);
> goto out_nolock;
> @@ -1214,22 +1210,21 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
> mmu_notifier_test_young(vma->vm_mm, address))
> referenced++;
> }
> - if (writable) {
> - if (referenced) {
> + if (!writable) {
> + result = SCAN_PAGE_RO;
> + } else if (!referenced || (unmapped && referenced < HPAGE_PMD_NR/2)) {
> + result = SCAN_LACK_REFERENCED_PAGE;
> + } else {
> result = SCAN_SUCCEED;
> ret = 1;

Shall fix the indentation for the above two statements?

> - } else {
> - result = SCAN_LACK_REFERENCED_PAGE;
> - }
> - } else {
> - result = SCAN_PAGE_RO;
> }
> out_unmap:
> pte_unmap_unlock(pte, ptl);
> if (ret) {
> node = khugepaged_find_target_node();
> /* collapse_huge_page will return with the mmap_sem released */
> - collapse_huge_page(mm, address, hpage, node, referenced);
> + collapse_huge_page(mm, address, hpage, node,
> + referenced, unmapped);
> }
> out:
> trace_mm_khugepaged_scan_pmd(mm, page, writable, referenced,

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