Re: [RFC PATCH v4 8/8] hugetlb: use new vma_lock for pmd sharing synchronization

Miaohe Lin
2022-07-28T06:51:55Z

On 2022/7/7 4:23, Mike Kravetz wrote:
> The new hugetlb vma lock (rw semaphore) is used to address this race:
>
> Faulting thread Unsharing thread
> ... ...
> ptep = huge_pte_offset()
> or
> ptep = huge_pte_alloc()
> ...
> i_mmap_lock_write
> lock page table
> ptep invalid <------------------------ huge_pmd_unshare()
> Could be in a previously unlock_page_table
> sharing process or worse i_mmap_unlock_write
> ...

Duplicated commit log here

> i_mmap_lock_write
> lock page table
> ptep invalid <------------------------ huge_pmd_unshare()
> Could be in a previously unlock_page_table
> sharing process or worse i_mmap_unlock_write
> ...

vs here?

> ptl = huge_pte_lock(ptep)
> get/update pte
> set_pte_at(pte, ptep)
>
> The vma_lock is used as follows:
> - During fault processing. the lock is acquired in read mode before
> doing a page table lock and allocation (huge_pte_alloc). The lock is
> held until code is finished with the page table entry (ptep).
> - The lock must be held in write mode whenever huge_pmd_unshare is
> called.
>
> Lock ordering issues come into play when unmapping a page from all
> vmas mapping the page. The i_mmap_rwsem must be held to search for the
> vmas, and the vma lock must be held before calling unmap which will
> call huge_pmd_unshare. This is done today in:
> - try_to_migrate_one and try_to_unmap_ for page migration and memory
> error handling. In these routines we 'try' to obtain the vma lock and
> fail to unmap if unsuccessful. Calling routines already deal with the
> failure of unmapping.
> - hugetlb_vmdelete_list for truncation and hole punch. This routine
> also tries to acquire the vma lock. If it fails, it skips the
> unmapping. However, we can not have file truncation or hole punch
> fail because of contention. After hugetlb_vmdelete_list, truncation
> and hole punch call remove_inode_hugepages. remove_inode_hugepages
> check for mapped pages and call hugetlb_unmap_file_page to unmap them.
> hugetlb_unmap_file_page is designed to drop locks and reacquire in the
> correct order to guarantee unmap success.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> fs/hugetlbfs/inode.c | 45 ++++++++++++++++++++
> mm/hugetlb.c | 76 ++++++++++++++++++++++++++++++----
> mm/memory.c | 2 +
> mm/rmap.c | 99 ++++++++++++++++++++++++++++----------------
> mm/userfaultfd.c | 9 +++-
> 5 files changed, 186 insertions(+), 45 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 0eac0ea2a245..be0a5073766f 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -459,6 +459,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
> struct folio *folio, pgoff_t index)
> {
> struct rb_root_cached *root = &mapping->i_mmap;
> + unsigned long skipped_vm_start;
> + struct mm_struct *skipped_mm;
> struct page *page = &folio->page;
> struct vm_area_struct *vma;
> unsigned long v_start;
> @@ -469,6 +471,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
> end = ((index + 1) * pages_per_huge_page(h));
>
> i_mmap_lock_write(mapping);
> +retry:
> + skipped_mm = NULL;
>
> vma_interval_tree_foreach(vma, root, start, end - 1) {
> v_start = vma_offset_start(vma, start);
> @@ -477,11 +481,48 @@ static void hugetlb_unmap_file_folio(struct hstate *h,
> if (!hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page))
> continue;
>
> + if (!hugetlb_vma_trylock_write(vma)) {
> + /*
> + * If we can not get vma lock, we need to drop
> + * immap_sema and take locks in order.
> + */
> + skipped_vm_start = vma->vm_start;
> + skipped_mm = vma->vm_mm;
> + /* grab mm-struct as we will be dropping i_mmap_sema */
> + mmgrab(skipped_mm);
> + break;
> + }
> +
> unmap_hugepage_range(vma, vma->vm_start + v_start, v_end,
> NULL, ZAP_FLAG_DROP_MARKER);
> + hugetlb_vma_unlock_write(vma);
> }
>
> i_mmap_unlock_write(mapping);
> +
> + if (skipped_mm) {
> + mmap_read_lock(skipped_mm);
> + mmdrop(skipped_mm);

Do we need to check &mm->mm_users == 0 here in case the address_space of corresponding process
has exited? In this case, mmdrop will drop the last reference and free the skipped_mm. So we will
use skipped_mm below after it's freed?

> + vma = find_vma(skipped_mm, skipped_vm_start);
> + if (!vma || vma->vm_file->f_mapping != mapping ||

If skipped_vm_start is unmapped and remapped as a anon vma before we taking the mmap_read_lock,
vma->vm_file will be NULL?

> + vma->vm_start != skipped_vm_start ||
> + !is_vm_hugetlb_page(vma)) {
> + mmap_read_unlock(skipped_mm);
> + goto retry;
> + }
> +
> + hugetlb_vma_lock_write(vma);
> + i_mmap_lock_write(mapping);
> + mmap_read_unlock(skipped_mm);
> +
> + v_start = vma_offset_start(vma, start);
> + v_end = vma_offset_end(vma, end);
> + unmap_hugepage_range(vma, vma->vm_start + v_start, v_end,
> + NULL, ZAP_FLAG_DROP_MARKER);
> + hugetlb_vma_unlock_write(vma);
> +
> + goto retry;
> + }
> }
>
> static void
> @@ -499,11 +540,15 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
> unsigned long v_start;
> unsigned long v_end;
>
> + if (!hugetlb_vma_trylock_write(vma))
> + continue;
> +
> v_start = vma_offset_start(vma, start);
> v_end = vma_offset_end(vma, end);
>
> unmap_hugepage_range(vma, vma->vm_start + v_start, v_end,
> NULL, zap_flags);
> + hugetlb_vma_unlock_write(vma);
> }
> }
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 2eca89bb08ab..8369db31df13 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4848,6 +4848,14 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> mmu_notifier_invalidate_range_start(&range);
> mmap_assert_write_locked(src);
> raw_write_seqcount_begin(&src->write_protect_seq);
> + } else {
> + /*
> + * For shared mappings the vma lock must be held before
> + * calling huge_pte_offset in the src vma. Otherwise, the
> + * returned ptep could go away if part of a shared pmd and
> + * another thread calls huge_pmd_unshare.
> + */
> + hugetlb_vma_lock_read(src_vma);
> }
>
> last_addr_mask = hugetlb_mask_last_page(h);
> @@ -4999,6 +5007,8 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> if (cow) {
> raw_write_seqcount_end(&src->write_protect_seq);
> mmu_notifier_invalidate_range_end(&range);
> + } else {
> + hugetlb_vma_unlock_read(src_vma);
> }
>
> return ret;
> @@ -5057,6 +5067,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
> mmu_notifier_invalidate_range_start(&range);
> last_addr_mask = hugetlb_mask_last_page(h);
> /* Prevent race with file truncation */
> + hugetlb_vma_lock_write(vma);
> i_mmap_lock_write(mapping);
> for (; old_addr < old_end; old_addr += sz, new_addr += sz) {
> src_pte = huge_pte_offset(mm, old_addr, sz);
> @@ -5088,6 +5099,7 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
> flush_tlb_range(vma, old_end - len, old_end);
> mmu_notifier_invalidate_range_end(&range);
> i_mmap_unlock_write(mapping);
> + hugetlb_vma_unlock_write(vma);
>
> return len + old_addr - old_end;
> }
> @@ -5392,9 +5404,30 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
> * may get SIGKILLed if it later faults.
> */
> if (outside_reserve) {
> + struct address_space *mapping = vma->vm_file->f_mapping;
> + pgoff_t idx;
> + u32 hash;
> +
> put_page(old_page);
> BUG_ON(huge_pte_none(pte));
> + /*
> + * Drop hugetlb_fault_mutex and vma_lock before
> + * unmapping. unmapping needs to hold vma_lock
> + * in write mode. Dropping vma_lock in read mode
> + * here is OK as COW mappings do not interact with
> + * PMD sharing.
> + *
> + * Reacquire both after unmap operation.
> + */
> + idx = vma_hugecache_offset(h, vma, haddr);
> + hash = hugetlb_fault_mutex_hash(mapping, idx);
> + mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> + hugetlb_vma_unlock_read(vma);
> +
> unmap_ref_private(mm, vma, old_page, haddr);
> +
> + hugetlb_vma_lock_read(vma);
> + mutex_lock(&hugetlb_fault_mutex_table[hash]);

We should respect the below lock order here?

hugetlb_fault_mutex
* hugetlbfs PageHuge() take locks in this order:
* hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
^^^^
* vma_lock (hugetlb specific lock for pmd_sharing)
^^^^
* mapping->i_mmap_rwsem (also used for hugetlb pmd sharing)
* page->flags PG_locked (lock_page)

> spin_lock(ptl);
> ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> if (likely(ptep &&
> @@ -5563,14 +5596,16 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
> };
>
> /*
> - * hugetlb_fault_mutex and i_mmap_rwsem must be
> + * vma_lock and hugetlb_fault_mutex must be
> * dropped before handling userfault. Reacquire
> * after handling fault to make calling code simpler.
> */
> + hugetlb_vma_unlock_read(vma);
> hash = hugetlb_fault_mutex_hash(mapping, idx);
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> ret = handle_userfault(&vmf, reason);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
> + hugetlb_vma_lock_read(vma);
>
> return ret;
> }
> @@ -5821,6 +5856,11 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>
> ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> if (ptep) {
> + /*
> + * Since we hold no locks, ptep could be stale. That is
> + * OK as we are only making decisions based on content and
> + * not actually modifying content here.
> + */
> entry = huge_ptep_get(ptep);
> if (unlikely(is_hugetlb_entry_migration(entry))) {
> migration_entry_wait_huge(vma, ptep);
> @@ -5828,23 +5868,35 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> } else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
> return VM_FAULT_HWPOISON_LARGE |
> VM_FAULT_SET_HINDEX(hstate_index(h));
> - } else {
> - ptep = huge_pte_alloc(mm, vma, haddr, huge_page_size(h));
> - if (!ptep)
> - return VM_FAULT_OOM;
> }
>
> - mapping = vma->vm_file->f_mapping;
> - idx = vma_hugecache_offset(h, vma, haddr);
> -
> /*
> * Serialize hugepage allocation and instantiation, so that we don't
> * get spurious allocation failures if two CPUs race to instantiate
> * the same page in the page cache.
> */
> + mapping = vma->vm_file->f_mapping;
> + idx = vma_hugecache_offset(h, vma, haddr);
> hash = hugetlb_fault_mutex_hash(mapping, idx);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
>
> + /*
> + * Acquire vma lock before calling huge_pte_alloc and hold
> + * until finished with ptep. This prevents huge_pmd_unshare from
> + * being called elsewhere and making the ptep no longer valid.
> + *
> + * ptep could have already be assigned via huge_pte_offset. That
> + * is OK, as huge_pte_alloc will return the same value unless
> + * something has changed.
> + */
> + hugetlb_vma_lock_read(vma);
> + ptep = huge_pte_alloc(mm, vma, haddr, huge_page_size(h));
> + if (!ptep) {
> + hugetlb_vma_unlock_read(vma);
> + mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> + return VM_FAULT_OOM;
> + }
> +
> entry = huge_ptep_get(ptep);
> /* PTE markers should be handled the same way as none pte */
> if (huge_pte_none_mostly(entry)) {
> @@ -5908,6 +5960,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> unlock_page(pagecache_page);
> put_page(pagecache_page);
> }
> + hugetlb_vma_unlock_read(vma);
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> return handle_userfault(&vmf, VM_UFFD_WP);
> }
> @@ -5951,6 +6004,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> put_page(pagecache_page);
> }
> out_mutex:
> + hugetlb_vma_unlock_read(vma);
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> /*
> * Generally it's safe to hold refcount during waiting page lock. But
> @@ -6413,8 +6467,9 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> flush_cache_range(vma, range.start, range.end);
>
> mmu_notifier_invalidate_range_start(&range);
> - last_addr_mask = hugetlb_mask_last_page(h);
> + hugetlb_vma_lock_write(vma);
> i_mmap_lock_write(vma->vm_file->f_mapping);
> + last_addr_mask = hugetlb_mask_last_page(h);
> for (; address < end; address += psize) {
> spinlock_t *ptl;
> ptep = huge_pte_offset(mm, address, psize);
> @@ -6513,6 +6568,7 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> * See Documentation/mm/mmu_notifier.rst
> */
> i_mmap_unlock_write(vma->vm_file->f_mapping);
> + hugetlb_vma_assert_locked(vma);

s/hugetlb_vma_assert_locked/hugetlb_vma_unlock_write/ ?

> mmu_notifier_invalidate_range_end(&range);
>
> return pages << h->order;
> @@ -6890,6 +6946,7 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
> pud_t *pud = pud_offset(p4d, addr);
>
> i_mmap_assert_write_locked(vma->vm_file->f_mapping);
> + hugetlb_vma_assert_locked(vma);
> BUG_ON(page_count(virt_to_page(ptep)) == 0);
> if (page_count(virt_to_page(ptep)) == 1)
> return 0;
> @@ -7271,6 +7328,7 @@ void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
> mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm,
> start, end);
> mmu_notifier_invalidate_range_start(&range);
> + hugetlb_vma_lock_write(vma);

hugetlb_vma_unlock_write is missed in hugetlb_unshare_all_pmds?

> i_mmap_lock_write(vma->vm_file->f_mapping);
> for (address = start; address < end; address += PUD_SIZE) {
> ptep = huge_pte_offset(mm, address, sz);
> diff --git a/mm/memory.c b/mm/memory.c
> index 8917bea2f0bc..3131766f9c7d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1693,10 +1693,12 @@ static void unmap_single_vma(struct mmu_gather *tlb,
> if (vma->vm_file) {
> zap_flags_t zap_flags = details ?
> details->zap_flags : 0;
> + hugetlb_vma_lock_write(vma);
> i_mmap_lock_write(vma->vm_file->f_mapping);
> __unmap_hugepage_range_final(tlb, vma, start, end,
> NULL, zap_flags);
> i_mmap_unlock_write(vma->vm_file->f_mapping);
> + hugetlb_vma_unlock_write(vma);
> }
> } else
> unmap_page_range(tlb, vma, start, end, details);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 64076c2a49c1..e1c19d86cea6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1557,24 +1557,38 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> * To call huge_pmd_unshare, i_mmap_rwsem must be
> * held in write mode. Caller needs to explicitly
> * do this outside rmap routines.
> + *
> + * We also must hold hugetlb vma_lock in write mode.
> + * Lock order dictates acquiring vma_lock BEFORE
> + * i_mmap_rwsem. We can only try lock here and fail
> + * if unsuccessful.
> */
> - VM_BUG_ON(!anon && !(flags & TTU_RMAP_LOCKED));
> - if (!anon && huge_pmd_unshare(mm, vma, address, pvmw.pte)) {
> - flush_tlb_range(vma, range.start, range.end);
> - mmu_notifier_invalidate_range(mm, range.start,
> - range.end);
> -
> - /*
> - * The ref count of the PMD page was dropped
> - * which is part of the way map counting
> - * is done for shared PMDs. Return 'true'
> - * here. When there is no other sharing,
> - * huge_pmd_unshare returns false and we will
> - * unmap the actual page and drop map count
> - * to zero.
> - */
> - page_vma_mapped_walk_done(&pvmw);
> - break;
> + if (!anon) {
> + VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
> + if (!hugetlb_vma_trylock_write(vma)) {
> + page_vma_mapped_walk_done(&pvmw);
> + ret = false;

We should break here?

Thanks.