Re: [PATCH 1/2] userfaultfd: handle zeropage moves by UFFDIO_MOVE

On Wed, Jan 24, 2024 at 4:13 PM Suren Baghdasaryan surenb@google.com wrote:

Current implementation of UFFDIO_MOVE fails to move zeropages and returns EBUSY when it encounters one. We can handle them by mapping a zeropage at the destination and clearing the mapping at the source. This is done both for ordinary and for huge zeropages.

I made a stupid mistake when formatting this patch and it says [PATCH 1/2] but it should be the only patch in the set. So, please do not look for [2/2]. Sorry about the confusion. Thanks, Suren.

Signed-off-by: Suren Baghdasaryan surenb@google.com

Applies cleanly over mm-unstable branch.

mm/huge_memory.c | 105 +++++++++++++++++++++++++++-------------------- mm/userfaultfd.c | 42 +++++++++++++++---- 2 files changed, 96 insertions(+), 51 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c index f40feb31b507..5dcc02c25e97 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2190,13 +2190,18 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm }

    src_page = pmd_page(src_pmdval);

•   if (unlikely(!PageAnonExclusive(src_page))) {
  

•           spin_unlock(src_ptl);
  

•           return -EBUSY;
  

•   }
  
  

•   src_folio = page_folio(src_page);
  

•   folio_get(src_folio);
  

•   if (!is_huge_zero_pmd(src_pmdval)) {
  

•           if (unlikely(!PageAnonExclusive(src_page))) {
  

•                   spin_unlock(src_ptl);
  

•                   return -EBUSY;
  

•           }
  

•           src_folio = page_folio(src_page);
  

•           folio_get(src_folio);
  

•   } else
  

•           src_folio = NULL;
  

•   spin_unlock(src_ptl);
  
    flush_cache_range(src_vma, src_addr, src_addr + HPAGE_PMD_SIZE);
  

@@ -2204,19 +2209,22 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm src_addr + HPAGE_PMD_SIZE); mmu_notifier_invalidate_range_start(&range);

•   folio_lock(src_folio);
  

•   if (src_folio) {
  

•           folio_lock(src_folio);
  
  

•   /*
  

•    * split_huge_page walks the anon_vma chain without the page
  

•    * lock. Serialize against it with the anon_vma lock, the page
  

•    * lock is not enough.
  

•    */
  

•   src_anon_vma = folio_get_anon_vma(src_folio);
  

•   if (!src_anon_vma) {
  

•           err = -EAGAIN;
  

•           goto unlock_folio;
  

•   }
  

•   anon_vma_lock_write(src_anon_vma);
  

•           /*
  

•            * split_huge_page walks the anon_vma chain without the page
  

•            * lock. Serialize against it with the anon_vma lock, the page
  

•            * lock is not enough.
  

•            */
  

•           src_anon_vma = folio_get_anon_vma(src_folio);
  

•           if (!src_anon_vma) {
  

•                   err = -EAGAIN;
  

•                   goto unlock_folio;
  

•           }
  

•           anon_vma_lock_write(src_anon_vma);
  

•   } else
  

•           src_anon_vma = NULL;
  
    dst_ptl = pmd_lockptr(mm, dst_pmd);
    double_pt_lock(src_ptl, dst_ptl);
  

@@ -2225,45 +2233,54 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm err = -EAGAIN; goto unlock_ptls; }

•   if (folio_maybe_dma_pinned(src_folio) ||
  

•       !PageAnonExclusive(&src_folio->page)) {
  

•           err = -EBUSY;
  

•           goto unlock_ptls;
  

•   }
  

•   if (src_folio) {
  

•           if (folio_maybe_dma_pinned(src_folio) ||
  

•               !PageAnonExclusive(&src_folio->page)) {
  

•                   err = -EBUSY;
  

•                   goto unlock_ptls;
  

•           }
  
  

•   if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
  

•       WARN_ON_ONCE(!folio_test_anon(src_folio))) {
  

•           err = -EBUSY;
  

•           goto unlock_ptls;
  

•   }
  

•           if (WARN_ON_ONCE(!folio_test_head(src_folio)) ||
  

•               WARN_ON_ONCE(!folio_test_anon(src_folio))) {
  

•                   err = -EBUSY;
  

•                   goto unlock_ptls;
  

•           }
  
  

•   folio_move_anon_rmap(src_folio, dst_vma);
  

•   WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
  

•           folio_move_anon_rmap(src_folio, dst_vma);
  

•           WRITE_ONCE(src_folio->index, linear_page_index(dst_vma, dst_addr));
  
  

•   src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
  

•   /* Folio got pinned from under us. Put it back and fail the move. */
  

•   if (folio_maybe_dma_pinned(src_folio)) {
  

•           set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
  

•           err = -EBUSY;
  

•           goto unlock_ptls;
  

•   }
  

•           src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
  

•           /* Folio got pinned from under us. Put it back and fail the move. */
  

•           if (folio_maybe_dma_pinned(src_folio)) {
  

•                   set_pmd_at(mm, src_addr, src_pmd, src_pmdval);
  

•                   err = -EBUSY;
  

•                   goto unlock_ptls;
  

•           }
  
  

•   _dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
  

•   /* Follow mremap() behavior and treat the entry dirty after the move */
  

•   _dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
  

•           _dst_pmd = mk_huge_pmd(&src_folio->page, dst_vma->vm_page_prot);
  

•           /* Follow mremap() behavior and treat the entry dirty after the move */
  

•           _dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
  

•   } else {
  

•           src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
  

•           _dst_pmd = mk_huge_pmd(src_page, dst_vma->vm_page_prot);
  

•   }
    set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
  
    src_pgtable = pgtable_trans_huge_withdraw(mm, src_pmd);
    pgtable_trans_huge_deposit(mm, dst_pmd, src_pgtable);
  

unlock_ptls: double_pt_unlock(src_ptl, dst_ptl);

•   anon_vma_unlock_write(src_anon_vma);
  

•   put_anon_vma(src_anon_vma);
  

•   if (src_anon_vma) {
  

•           anon_vma_unlock_write(src_anon_vma);
  

•           put_anon_vma(src_anon_vma);
  

•   }
  

unlock_folio: /* unblock rmap walks */

•   folio_unlock(src_folio);
  

•   if (src_folio)
  

•           folio_unlock(src_folio);
    mmu_notifier_invalidate_range_end(&range);
  

•   folio_put(src_folio);
  

•   if (src_folio)
  

•           folio_put(src_folio);
    return err;
  

} #endif /* CONFIG_USERFAULTFD */ diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 3548b3e31a97..5fbf4da15c5c 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -959,6 +959,31 @@ static int move_swap_pte(struct mm_struct *mm, return 0; }

+static int move_zeropage_pte(struct mm_struct *mm,

•                        struct vm_area_struct *dst_vma,
  

•                        struct vm_area_struct *src_vma,
  

•                        unsigned long dst_addr, unsigned long src_addr,
  

•                        pte_t *dst_pte, pte_t *src_pte,
  

•                        pte_t orig_dst_pte, pte_t orig_src_pte,
  

•                        spinlock_t *dst_ptl, spinlock_t *src_ptl)
  

+{

•   pte_t zero_pte;
  

•   double_pt_lock(dst_ptl, src_ptl);
  

•   if (!pte_same(ptep_get(src_pte), orig_src_pte) ||
  

•       !pte_same(ptep_get(dst_pte), orig_dst_pte))
  

•           return -EAGAIN;
  

•   zero_pte = pte_mkspecial(pfn_pte(my_zero_pfn(dst_addr),
  

•                                    dst_vma->vm_page_prot));
  

•   ptep_clear_flush(src_vma, src_addr, src_pte);
  

•   set_pte_at(mm, dst_addr, dst_pte, zero_pte);
  

•   double_pt_unlock(dst_ptl, src_ptl);
  

•   return 0;
  

+}

/*

• The mmap_lock for reading is held by the caller. Just move the page
• from src_pmd to dst_pmd if possible, and return true if succeeded

@@ -1041,6 +1066,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, }

    if (pte_present(orig_src_pte)) {

•           if (is_zero_pfn(pte_pfn(orig_src_pte))) {
  

•                   err = move_zeropage_pte(mm, dst_vma, src_vma,
  

•                                          dst_addr, src_addr, dst_pte, src_pte,
  

•                                          orig_dst_pte, orig_src_pte,
  

•                                          dst_ptl, src_ptl);
  

•                   goto out;
  

•           }
  

•           /*
             * Pin and lock both source folio and anon_vma. Since we are in
             * RCU read section, we can't block, so on contention have to
  

@@ -1404,19 +1437,14 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm, err = -ENOENT; break; }

•                   /* Avoid moving zeropages for now */
  

•                   if (is_huge_zero_pmd(*src_pmd)) {
  

•                           spin_unlock(ptl);
  

•                           err = -EBUSY;
  

•                           break;
  

•                   }
  
                    /* Check if we can move the pmd without splitting it. */
                    if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
                        !pmd_none(dst_pmdval)) {
                            struct folio *folio = pfn_folio(pmd_pfn(*src_pmd));
  
  

•                           if (!folio || !PageAnonExclusive(&folio->page)) {
  

•                           if (!folio || (!is_huge_zero_page(&folio->page) &&
  

•                                          !PageAnonExclusive(&folio->page))) {
                                    spin_unlock(ptl);
                                    err = -EBUSY;
                                    break;
  

-- 2.43.0.429.g432eaa2c6b-goog