On Fri, Jun 3, 2022 at 5:40 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 THP. 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.
>
> Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
> ---
> include/trace/events/huge_memory.h | 1 +
> mm/internal.h | 1 +
> mm/khugepaged.c | 32 ++++++++++++++++++++++++++----
> mm/rmap.c | 15 ++++++++++++--
> 4 files changed, 43 insertions(+), 6 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/internal.h b/mm/internal.h
> index 6e14749ad1e5..f768c7fae668 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -188,6 +188,7 @@ extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason
> /*
> * in mm/rmap.c:
> */
> +pmd_t *mm_find_pmd_raw(struct mm_struct *mm, unsigned long address);
> extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
>
> /*
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index cc3d6fb446d5..7a914ca19e96 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,
> @@ -901,6 +902,31 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
> return 0;
> }
>
> +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_raw(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_FAIL;
khugepaged doesn't handle pmd_bad before, IIRC it may just return
SCAN_SUCCEED if everything else is good? It is fine to add it, but it
may be better to return SCAN_PMD_NULL?
> + return SCAN_SUCCEED;
> +}
> +
> /*
> * Bring missing pages in from swap, to complete THP collapse.
> * Only done if khugepaged_scan_pmd believes it is worthwhile.
> @@ -1146,11 +1172,9 @@ static int khugepaged_scan_pmd(struct mm_struct *mm,
>
> 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)
There are a couple of other callsites for mm_find_pmd(), you may need
to change all of them to find_pmd_or_thp_or_none() for MADV_COLLAPSE
since khugepaged may collapse the area before MADV_COLLAPSE
reacquiring mmap_lock IIUC and MADV_COLLAPSE does care this case. It
is fine w/o MADV_COLLAPSE since khupaged doesn't care if it is PMD
mapped or not.
So it may be better to move this patch right before MADV_COLLAPSE is introduced?
> goto out;
> - }
>
> memset(khugepaged_node_load, 0, sizeof(khugepaged_node_load));
> pte = pte_offset_map_lock(mm, pmd, address, &ptl);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 04fac1af870b..c9979c6ad7a1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -767,13 +767,12 @@ unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
> return vma_address(page, vma);
> }
>
> -pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> +pmd_t *mm_find_pmd_raw(struct mm_struct *mm, unsigned long address)
May be better to have some notes for mm_find_pmd_raw() and mm_find_pmd().
> {
> 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))
> @@ -788,6 +787,18 @@ pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> goto out;
>
> pmd = pmd_offset(pud, address);
> +out:
> + return pmd;
> +}
> +
> +pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address)
> +{
> + pmd_t pmde;
> + pmd_t *pmd;
> +
> + pmd = mm_find_pmd_raw(mm, address);
> + 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
> --
> 2.36.1.255.ge46751e96f-goog
>
