On Wed, Jul 6, 2022 at 5:06 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
>
> When scanning an anon pmd to see if it's eligible for collapse, return
> SCAN_PMD_MAPPED if the pmd already maps a hugepage. Note that
> SCAN_PMD_MAPPED is different from SCAN_PAGE_COMPOUND used in the
> file-collapse path, since the latter might identify pte-mapped compound
> pages. This is required by MADV_COLLAPSE which necessarily needs to
> know what hugepage-aligned/sized regions are already pmd-mapped.
>
> In order to determine if a pmd already maps a hugepage, refactor
> mm_find_pmd():
>
> Return mm_find_pmd() to it's pre-commit f72e7dcdd252 ("mm: let mm_find_pmd
> fix buggy race with THP fault") behavior. ksm was the only caller that
> explicitly wanted a pte-mapping pmd, so open code the pte-mapping logic
> there (pmd_present() and pmd_trans_huge() checks).
>
> Undo revert change in commit f72e7dcdd252 ("mm: let mm_find_pmd fix buggy race
> with THP fault") that open-coded split_huge_pmd_address() pmd lookup and
> use mm_find_pmd() instead.
>
> Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
> include/trace/events/huge_memory.h | 1 +
> mm/huge_memory.c | 18 +--------
> mm/internal.h | 2 +-
> mm/khugepaged.c | 60 ++++++++++++++++++++++++------
> mm/ksm.c | 10 +++++
> mm/rmap.c | 15 +++-----
> 6 files changed, 67 insertions(+), 39 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index d651f3437367..55392bf30a03 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -11,6 +11,7 @@
> EM( SCAN_FAIL, "failed") \
> EM( SCAN_SUCCEED, "succeeded") \
> EM( SCAN_PMD_NULL, "pmd_null") \
> + EM( SCAN_PMD_MAPPED, "page_pmd_mapped") \
> EM( SCAN_EXCEED_NONE_PTE, "exceed_none_pte") \
> EM( SCAN_EXCEED_SWAP_PTE, "exceed_swap_pte") \
> EM( SCAN_EXCEED_SHARED_PTE, "exceed_shared_pte") \
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 4fbe43dc1568..fb76db6c703e 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2363,25 +2363,11 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> void split_huge_pmd_address(struct vm_area_struct *vma, unsigned long address,
> bool freeze, struct folio *folio)
> {
> - pgd_t *pgd;
> - p4d_t *p4d;
> - pud_t *pud;
> - pmd_t *pmd;
> + pmd_t *pmd = mm_find_pmd(vma->vm_mm, address);
>
> - pgd = pgd_offset(vma->vm_mm, address);
> - if (!pgd_present(*pgd))
> + if (!pmd)
> return;
>
> - p4d = p4d_offset(pgd, address);
> - if (!p4d_present(*p4d))
> - return;
> -
> - pud = pud_offset(p4d, address);
> - if (!pud_present(*pud))
> - return;
> -
> - pmd = pmd_offset(pud, address);
> -
> __split_huge_pmd(vma, pmd, address, freeze, folio);
> }
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 6e14749ad1e5..ef8c23fb678f 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -188,7 +188,7 @@ extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason
> /*
> * in mm/rmap.c:
> */
> -extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
> +pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
>
> /*
> * in mm/page_alloc.c
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b0e20db3f805..c7a09cc9a0e8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -28,6 +28,7 @@ enum scan_result {
> SCAN_FAIL,
> SCAN_SUCCEED,
> SCAN_PMD_NULL,
> + SCAN_PMD_MAPPED,
> SCAN_EXCEED_NONE_PTE,
> SCAN_EXCEED_SWAP_PTE,
> SCAN_EXCEED_SHARED_PTE,
> @@ -871,6 +872,45 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> return SCAN_SUCCEED;
> }
>
> +static int find_pmd_or_thp_or_none(struct mm_struct *mm,
> + unsigned long address,
> + pmd_t **pmd)
> +{
> + pmd_t pmde;
> +
> + *pmd = mm_find_pmd(mm, address);
> + if (!*pmd)
> + return SCAN_PMD_NULL;
> +
> + pmde = pmd_read_atomic(*pmd);
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> + /* See comments in pmd_none_or_trans_huge_or_clear_bad() */
> + barrier();
> +#endif
> + if (!pmd_present(pmde))
> + return SCAN_PMD_NULL;
> + if (pmd_trans_huge(pmde))
> + return SCAN_PMD_MAPPED;
> + if (pmd_bad(pmde))
> + return SCAN_PMD_NULL;
> + return SCAN_SUCCEED;
> +}
> +
> +static int check_pmd_still_valid(struct mm_struct *mm,
> + unsigned long address,
> + pmd_t *pmd)
> +{
> + pmd_t *new_pmd;
> + int result = find_pmd_or_thp_or_none(mm, address, &new_pmd);
> +
> + if (result != SCAN_SUCCEED)
> + return result;
> + if (new_pmd != pmd)
> + return SCAN_FAIL;
> + return SCAN_SUCCEED;
> +}
> +
> /*
> * Bring missing pages in from swap, to complete THP collapse.
> * Only done if khugepaged_scan_pmd believes it is worthwhile.
> @@ -982,9 +1022,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> goto out_nolock;
> }
>
> - pmd = mm_find_pmd(mm, address);
> - if (!pmd) {
> - result = SCAN_PMD_NULL;
> + result = find_pmd_or_thp_or_none(mm, address, &pmd);
> + if (result != SCAN_SUCCEED) {
> mmap_read_unlock(mm);
> goto out_nolock;
> }
> @@ -1012,7 +1051,8 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
> if (result != SCAN_SUCCEED)
> goto out_up_write;
> /* check if the pmd is still valid */
> - if (mm_find_pmd(mm, address) != pmd)
> + result = check_pmd_still_valid(mm, address, pmd);
> + if (result != SCAN_SUCCEED)
> goto out_up_write;
>
> anon_vma_lock_write(vma->anon_vma);
> @@ -1115,11 +1155,9 @@ static int khugepaged_scan_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
>
> VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>
> - pmd = mm_find_pmd(mm, address);
> - if (!pmd) {
> - result = SCAN_PMD_NULL;
> + result = find_pmd_or_thp_or_none(mm, address, &pmd);
> + if (result != SCAN_SUCCEED)
> goto out;
> - }
>
> memset(cc->node_load, 0, sizeof(cc->node_load));
> pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> @@ -1373,8 +1411,7 @@ void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
> if (!PageHead(hpage))
> goto drop_hpage;
>
> - pmd = mm_find_pmd(mm, haddr);
> - if (!pmd)
> + if (find_pmd_or_thp_or_none(mm, haddr, &pmd) != SCAN_SUCCEED)
> goto drop_hpage;
>
> start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl);
> @@ -1492,8 +1529,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
> if (vma->vm_end < addr + HPAGE_PMD_SIZE)
> continue;
> mm = vma->vm_mm;
> - pmd = mm_find_pmd(mm, addr);
> - if (!pmd)
> + if (find_pmd_or_thp_or_none(mm, addr, &pmd) != SCAN_SUCCEED)
> continue;
> /*
> * We need exclusive mmap_lock to retract page table.
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 075123602bd0..3e0a0a42fa1f 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -1136,6 +1136,7 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
> {
> struct mm_struct *mm = vma->vm_mm;
> pmd_t *pmd;
> + pmd_t pmde;
> pte_t *ptep;
> pte_t newpte;
> spinlock_t *ptl;
> @@ -1150,6 +1151,15 @@ static int replace_page(struct vm_area_struct *vma, struct page *page,
> pmd = mm_find_pmd(mm, addr);
> if (!pmd)
> goto out;
> + /*
> + * Some THP functions use the sequence pmdp_huge_clear_flush(), set_pmd_at()
> + * without holding anon_vma lock for write. So when looking for a
> + * genuine pmde (in which to find pte), test present and !THP together.
> + */
> + pmde = *pmd;
> + barrier();
> + if (!pmd_present(pmde) || pmd_trans_huge(pmde))
> + goto out;
>
> mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm, addr,
> addr + PAGE_SIZE);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index edc06c52bc82..af775855e58f 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -767,13 +767,17 @@ unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
> return vma_address(page, vma);
> }
>
> +/*
> + * Returns the actual pmd_t* where we expect 'address' to be mapped from, or
> + * NULL if it doesn't exist. No guarantees / checks on what the pmd_t*
> + * represents.
> + */
> pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> {
> pgd_t *pgd;
> p4d_t *p4d;
> pud_t *pud;
> pmd_t *pmd = NULL;
> - pmd_t pmde;
>
> pgd = pgd_offset(mm, address);
> if (!pgd_present(*pgd))
> @@ -788,15 +792,6 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> goto out;
>
> pmd = pmd_offset(pud, address);
> - /*
> - * Some THP functions use the sequence pmdp_huge_clear_flush(), set_pmd_at()
> - * without holding anon_vma lock for write. So when looking for a
> - * genuine pmde (in which to find pte), test present and !THP together.
> - */
> - pmde = *pmd;
> - barrier();
> - if (!pmd_present(pmde) || pmd_trans_huge(pmde))
> - pmd = NULL;
> out:
> return pmd;
> }
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>
