Re: [PATCH v2 4/4] mm: Avoid splitting pmd for lazyfree pmd-mapped THP in try_to_unmap

Lance Yang
2025-01-14T06:32:19Z

On Tue, Jan 14, 2025 at 12:09 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Tue, Jan 14, 2025 at 4:40 PM Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
> >
> >
> >
> > On 2025/1/13 11:39, Barry Song wrote:
> > > From: Barry Song <v-songbaohua@oppo.com>
> > >
> > > The try_to_unmap_one() function currently handles PMD-mapped THPs
> > > inefficiently. It first splits the PMD into PTEs, copies the dirty
> > > state from the PMD to the PTEs, iterates over the PTEs to locate
> > > the dirty state, and then marks the THP as swap-backed. This process
> > > involves unnecessary PMD splitting and redundant iteration. Instead,
> > > this functionality can be efficiently managed in
> > > __discard_anon_folio_pmd_locked(), avoiding the extra steps and
> > > improving performance.
> > >
> > > The following microbenchmark redirties folios after invoking MADV_FREE,
> > > then measures the time taken to perform memory reclamation (actually
> > > set those folios swapbacked again) on the redirtied folios.
> > >
> > > #include <stdio.h>
> > > #include <sys/mman.h>
> > > #include <string.h>
> > > #include <time.h>
> > >
> > > #define SIZE 128*1024*1024 // 128 MB
> > >
> > > int main(int argc, char *argv[])
> > > {
> > > while(1) {
> > > volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
> > > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> > >
> > > memset((void *)p, 1, SIZE);
> > > madvise((void *)p, SIZE, MADV_FREE);
> > > /* redirty after MADV_FREE */
> > > memset((void *)p, 1, SIZE);
> > >
> > > clock_t start_time = clock();
> > > madvise((void *)p, SIZE, MADV_PAGEOUT);
> > > clock_t end_time = clock();
> > >
> > > double elapsed_time = (double)(end_time - start_time) / CLOCKS_PER_SEC;
> > > printf("Time taken by reclamation: %f seconds\n", elapsed_time);
> > >
> > > munmap((void *)p, SIZE);
> > > }
> > > return 0;
> > > }
> > >
> > > Testing results are as below,
> > > w/o patch:
> > > ~ # ./a.out
> > > Time taken by reclamation: 0.007300 seconds
> > > Time taken by reclamation: 0.007226 seconds
> > > Time taken by reclamation: 0.007295 seconds
> > > Time taken by reclamation: 0.007731 seconds
> > > Time taken by reclamation: 0.007134 seconds
> > > Time taken by reclamation: 0.007285 seconds
> > > Time taken by reclamation: 0.007720 seconds
> > > Time taken by reclamation: 0.007128 seconds
> > > Time taken by reclamation: 0.007710 seconds
> > > Time taken by reclamation: 0.007712 seconds
> > > Time taken by reclamation: 0.007236 seconds
> > > Time taken by reclamation: 0.007690 seconds
> > > Time taken by reclamation: 0.007174 seconds
> > > Time taken by reclamation: 0.007670 seconds
> > > Time taken by reclamation: 0.007169 seconds
> > > Time taken by reclamation: 0.007305 seconds
> > > Time taken by reclamation: 0.007432 seconds
> > > Time taken by reclamation: 0.007158 seconds
> > > Time taken by reclamation: 0.007133 seconds
> > > …
> > >
> > > w/ patch
> > >
> > > ~ # ./a.out
> > > Time taken by reclamation: 0.002124 seconds
> > > Time taken by reclamation: 0.002116 seconds
> > > Time taken by reclamation: 0.002150 seconds
> > > Time taken by reclamation: 0.002261 seconds
> > > Time taken by reclamation: 0.002137 seconds
> > > Time taken by reclamation: 0.002173 seconds
> > > Time taken by reclamation: 0.002063 seconds
> > > Time taken by reclamation: 0.002088 seconds
> > > Time taken by reclamation: 0.002169 seconds
> > > Time taken by reclamation: 0.002124 seconds
> > > Time taken by reclamation: 0.002111 seconds
> > > Time taken by reclamation: 0.002224 seconds
> > > Time taken by reclamation: 0.002297 seconds
> > > Time taken by reclamation: 0.002260 seconds
> > > Time taken by reclamation: 0.002246 seconds
> > > Time taken by reclamation: 0.002272 seconds
> > > Time taken by reclamation: 0.002277 seconds
> > > Time taken by reclamation: 0.002462 seconds
> > > …
> > >
> > > This patch significantly speeds up try_to_unmap_one() by allowing it
> > > to skip redirtied THPs without splitting the PMD.
> > >
> > > Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > > Suggested-by: Lance Yang <ioworker0@gmail.com>
> > > Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> > > ---
> > > mm/huge_memory.c | 17 ++++++++++++++---
> > > mm/rmap.c | 11 ++++++++++-
> > > 2 files changed, 24 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index 3d3ebdc002d5..aea49f7125f1 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -3070,8 +3070,12 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
> > > int ref_count, map_count;
> > > pmd_t orig_pmd = *pmdp;
> > >
> > > - if (folio_test_dirty(folio) || pmd_dirty(orig_pmd))
> > > + if (pmd_dirty(orig_pmd))
> > > + folio_set_dirty(folio);
> > > + if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
> > > + folio_set_swapbacked(folio);
> > > return false;
> > > + }
> > >
> > > orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
> > >
> > > @@ -3098,8 +3102,15 @@ static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
> > > *
> > > * The only folio refs must be one from isolation plus the rmap(s).
> > > */
> > > - if (folio_test_dirty(folio) || pmd_dirty(orig_pmd) ||
> > > - ref_count != map_count + 1) {
> > > + if (pmd_dirty(orig_pmd))
> > > + folio_set_dirty(folio);
> > > + if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
> > > + folio_set_swapbacked(folio);
> > > + set_pmd_at(mm, addr, pmdp, orig_pmd);
> > > + return false;
> > > + }
> > > +
> > > + if (ref_count != map_count + 1) {
> > > set_pmd_at(mm, addr, pmdp, orig_pmd);
> > > return false;
> > > }
> > > diff --git a/mm/rmap.c b/mm/rmap.c
> > > index 3ef659310797..02c4e4b2cd7b 100644
> > > --- a/mm/rmap.c
> > > +++ b/mm/rmap.c
> > > @@ -1671,7 +1671,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > > DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> > > pte_t pteval;
> > > struct page *subpage;
> > > - bool anon_exclusive, ret = true;
> > > + bool anon_exclusive, lazyfree, ret = true;
> > > struct mmu_notifier_range range;
> > > enum ttu_flags flags = (enum ttu_flags)(long)arg;
> > > int nr_pages = 1;
> > > @@ -1724,9 +1724,18 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> > > }
> > >
> > > if (!pvmw.pte) {
> > > + lazyfree = folio_test_anon(folio) && !folio_test_swapbacked(folio);
> >
> > You've checked lazyfree here, so can we remove the duplicate check in
> > unmap_huge_pmd_locked()? Then the code should be:
> >
> > if (lazyfree && unmap_huge_pmd_locked(...))
> > goto walk_done;
>
>
> right. it seems unmap_huge_pmd_locked() only handles lazyfree pmd-mapped
> thp. so i guess the code could be:

Yep, it would be better to remove the duplicate check in
unmap_huge_pmd_locked(), and we can extend unmap_huge_pmd_locked()
in the future if needed.

Thanks,
Lance

>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index aea49f7125f1..c4c3a7896de4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3131,11 +3131,10 @@ bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
> VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
> VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
> VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
> + VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
> + VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
>
> - if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
> - return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
> -
> - return false;
> + return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
> }
>
> static void remap_page(struct folio *folio, unsigned long nr, int flags)
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 02c4e4b2cd7b..72907eb1b8fe 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1671,7 +1671,7 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
> pte_t pteval;
> struct page *subpage;
> - bool anon_exclusive, lazyfree, ret = true;
> + bool anon_exclusive, ret = true;
> struct mmu_notifier_range range;
> enum ttu_flags flags = (enum ttu_flags)(long)arg;
> int nr_pages = 1;
> @@ -1724,18 +1724,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> }
>
> if (!pvmw.pte) {
> - lazyfree = folio_test_anon(folio) && !folio_test_swapbacked(folio);
> -
> - if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> - folio))
> - goto walk_done;
> - /*
> - * unmap_huge_pmd_locked has either already marked
> - * the folio as swap-backed or decided to retain it
> - * due to GUP or speculative references.
> - */
> - if (lazyfree)
> + if (folio_test_anon(folio) && !folio_test_swapbacked(folio)) {
> + if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, folio))
> + goto walk_done;
> + /*
> + * unmap_huge_pmd_locked has either already marked
> + * the folio as swap-backed or decided to retain it
> + * due to GUP or speculative references.
> + */
> goto walk_abort;
> + }
>
> if (flags & TTU_SPLIT_HUGE_PMD) {
> /*
>
> >
> > > if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
> > > folio))
> > > goto walk_done;
> > > + /*
> > > + * unmap_huge_pmd_locked has either already marked
> > > + * the folio as swap-backed or decided to retain it
> > > + * due to GUP or speculative references.
> > > + */
> > > + if (lazyfree)
> > > + goto walk_abort;
> > >
> > > if (flags & TTU_SPLIT_HUGE_PMD) {
> > > /*