Commit ffe945e6 authored by Kirill A. Shutemov's avatar Kirill A. Shutemov Committed by Linus Torvalds
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khugepaged: do not stop collapse if less than half PTEs are referenced 



__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.

Fixes: 0db501f7 ("mm, thp: convert from optimistic swapin collapsing to conservative")
Signed-off-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Tested-by: default avatarZi Yan <ziy@nvidia.com>
Reviewed-by: default avatarWilliam Kucharski <william.kucharski@oracle.com>
Reviewed-by: default avatarZi Yan <ziy@nvidia.com>
Acked-by: default avatarYang Shi <yang.shi@linux.alibaba.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Link: http://lkml.kernel.org/r/20200416160026.16538-3-kirill.shutemov@linux.intel.com


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e0c13f97

mm/khugepaged.c

+11 −16
Original line number Diff line number Diff line
@@ -899,11 +899,6 @@ 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) {
@@ -943,7 +938,7 @@ 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;
@@ -1000,7 +995,8 @@ 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;
@@ -1233,22 +1229,21 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, 
		    mmu_notifier_test_young(vma->vm_mm, address))

			referenced++;

	}

	if (writable) {

		if (referenced) {

			result = SCAN_SUCCEED;

			ret = 1;

		} else {

	if (!writable) {

		result = SCAN_PAGE_RO;

	} else if (!referenced || (unmapped && referenced < HPAGE_PMD_NR/2)) {

		result = SCAN_LACK_REFERENCED_PAGE;

		}

	} else {

		result = SCAN_PAGE_RO;

		result = SCAN_SUCCEED;

		ret = 1;

	}

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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