Re: [PATCH 2/8] hugetlbfs: revert use i_mmap_rwsem for more pmd sharing synchronization

Miaohe Lin
2022-08-27T02:59:55Z

On 2022/8/25 1:57, Mike Kravetz wrote:
> Commit c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
> synchronization") added code to take i_mmap_rwsem in read mode for the
> duration of fault processing. However, this has been shown to cause
> performance/scaling issues. Revert the code and go back to only taking
> the semaphore in huge_pmd_share during the fault path.
>
> Keep the code that takes i_mmap_rwsem in write mode before calling
> try_to_unmap as this is required if huge_pmd_unshare is called.
>
> NOTE: Reverting this code does expose the following race condition.
>
> 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
> ...
> ptl = huge_pte_lock(ptep)
> get/update pte
> set_pte_at(pte, ptep)
>
> It is unknown if the above race was ever experienced by a user. It was
> discovered via code inspection when initially addressed.
>
> In subsequent patches, a new synchronization mechanism will be added to
> coordinate pmd sharing and eliminate this race.
>
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

LGTM. Thanks.

Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>

Thanks,
Miaohe Lin

> ---
> fs/hugetlbfs/inode.c | 2 --
> mm/hugetlb.c | 77 +++++++-------------------------------------
> mm/rmap.c | 8 +----
> mm/userfaultfd.c | 11 ++-----
> 4 files changed, 15 insertions(+), 83 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index a32031e751d1..dfb735a91bbb 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -467,9 +467,7 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
> if (unlikely(folio_mapped(folio))) {
> BUG_ON(truncate_op);
>
> - mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> i_mmap_lock_write(mapping);
> - mutex_lock(&hugetlb_fault_mutex_table[hash]);
> hugetlb_vmdelete_list(&mapping->i_mmap,
> index * pages_per_huge_page(h),
> (index + 1) * pages_per_huge_page(h),
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 70bc7f867bc0..95c6f9a5bbf0 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4770,7 +4770,6 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> struct hstate *h = hstate_vma(src_vma);
> unsigned long sz = huge_page_size(h);
> unsigned long npages = pages_per_huge_page(h);
> - struct address_space *mapping = src_vma->vm_file->f_mapping;
> struct mmu_notifier_range range;
> unsigned long last_addr_mask;
> int ret = 0;
> @@ -4782,14 +4781,6 @@ 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 i_mmap_rwsem must be held to call
> - * huge_pte_alloc, otherwise the returned ptep could go
> - * away if part of a shared pmd and another thread calls
> - * huge_pmd_unshare.
> - */
> - i_mmap_lock_read(mapping);
> }
>
> last_addr_mask = hugetlb_mask_last_page(h);
> @@ -4937,8 +4928,6 @@ 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 {
> - i_mmap_unlock_read(mapping);
> }
>
> return ret;
> @@ -5347,30 +5336,9 @@ 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 i_mmap_rwsem before
> - * unmapping. unmapping needs to hold i_mmap_rwsem
> - * in write mode. Dropping i_mmap_rwsem 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]);
> - i_mmap_unlock_read(mapping);
> -
> unmap_ref_private(mm, vma, old_page, haddr);
> -
> - i_mmap_lock_read(mapping);
> - mutex_lock(&hugetlb_fault_mutex_table[hash]);
> spin_lock(ptl);
> ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
> if (likely(ptep &&
> @@ -5538,9 +5506,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
> */
> hash = hugetlb_fault_mutex_hash(mapping, idx);
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - i_mmap_unlock_read(mapping);
> ret = handle_userfault(&vmf, reason);
> - i_mmap_lock_read(mapping);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
>
> return ret;
> @@ -5772,11 +5738,6 @@ 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);
> @@ -5784,31 +5745,20 @@ 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;
> }
>
> - /*
> - * Acquire i_mmap_rwsem 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.
> - */
> mapping = vma->vm_file->f_mapping;
> - i_mmap_lock_read(mapping);
> - ptep = huge_pte_alloc(mm, vma, haddr, huge_page_size(h));
> - if (!ptep) {
> - i_mmap_unlock_read(mapping);
> - return VM_FAULT_OOM;
> - }
> + 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.
> */
> - idx = vma_hugecache_offset(h, vma, haddr);
> hash = hugetlb_fault_mutex_hash(mapping, idx);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
>
> @@ -5873,7 +5823,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> put_page(pagecache_page);
> }
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - i_mmap_unlock_read(mapping);
> return handle_userfault(&vmf, VM_UFFD_WP);
> }
>
> @@ -5917,7 +5866,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> }
> out_mutex:
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - i_mmap_unlock_read(mapping);
> /*
> * Generally it's safe to hold refcount during waiting page lock. But
> * here we just wait to defer the next page fault to avoid busy loop and
> @@ -6758,12 +6706,10 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> * Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
> * and returns the corresponding pte. While this is not necessary for the
> * !shared pmd case because we can allocate the pmd later as well, it makes the
> - * code much cleaner.
> - *
> - * This routine must be called with i_mmap_rwsem held in at least read mode if
> - * sharing is possible. For hugetlbfs, this prevents removal of any page
> - * table entries associated with the address space. This is important as we
> - * are setting up sharing based on existing page table entries (mappings).
> + * code much cleaner. pmd allocation is essential for the shared case because
> + * pud has to be populated inside the same i_mmap_rwsem section - otherwise
> + * racing tasks could either miss the sharing (see huge_pte_offset) or select a
> + * bad pmd for sharing.
> */
> pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
> unsigned long addr, pud_t *pud)
> @@ -6777,7 +6723,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
> pte_t *pte;
> spinlock_t *ptl;
>
> - i_mmap_assert_locked(mapping);
> + i_mmap_lock_read(mapping);
> vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
> if (svma == vma)
> continue;
> @@ -6807,6 +6753,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
> spin_unlock(ptl);
> out:
> pte = (pte_t *)pmd_alloc(mm, pud, addr);
> + i_mmap_unlock_read(mapping);
> return pte;
> }
>
> @@ -6817,7 +6764,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, struct vm_area_struct *vma,
> * indicated by page_count > 1, unmap is achieved by clearing pud and
> * decrementing the ref count. If count == 1, the pte page is not shared.
> *
> - * Called with page table lock held and i_mmap_rwsem held in write mode.
> + * Called with page table lock held.
> *
> * returns: 1 successfully unmapped a shared pte page
> * 0 the underlying pte page is not shared, or it is the last user
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 7dc6d77ae865..ad9c97c6445c 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -23,10 +23,9 @@
> * inode->i_rwsem (while writing or truncating, not reading or faulting)
> * mm->mmap_lock
> * mapping->invalidate_lock (in filemap_fault)
> - * page->flags PG_locked (lock_page) * (see hugetlbfs below)
> + * page->flags PG_locked (lock_page)
> * hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share)
> * mapping->i_mmap_rwsem
> - * hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
> * anon_vma->rwsem
> * mm->page_table_lock or pte_lock
> * swap_lock (in swap_duplicate, swap_info_get)
> @@ -45,11 +44,6 @@
> * anon_vma->rwsem,mapping->i_mmap_rwsem (memory_failure, collect_procs_anon)
> * ->tasklist_lock
> * pte map lock
> - *
> - * * hugetlbfs PageHuge() pages take locks in this order:
> - * mapping->i_mmap_rwsem
> - * hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
> - * page->flags PG_locked (lock_page)
> */
>
> #include <linux/mm.h>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 7327b2573f7c..7707f2664adb 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -377,14 +377,10 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> BUG_ON(dst_addr >= dst_start + len);
>
> /*
> - * Serialize via i_mmap_rwsem and hugetlb_fault_mutex.
> - * i_mmap_rwsem ensures the dst_pte remains valid even
> - * in the case of shared pmds. fault mutex prevents
> - * races with other faulting threads.
> + * Serialize via hugetlb_fault_mutex.
> */
> - mapping = dst_vma->vm_file->f_mapping;
> - i_mmap_lock_read(mapping);
> idx = linear_page_index(dst_vma, dst_addr);
> + mapping = dst_vma->vm_file->f_mapping;
> hash = hugetlb_fault_mutex_hash(mapping, idx);
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
>
> @@ -392,7 +388,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> dst_pte = huge_pte_alloc(dst_mm, dst_vma, dst_addr, vma_hpagesize);
> if (!dst_pte) {
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - i_mmap_unlock_read(mapping);
> goto out_unlock;
> }
>
> @@ -400,7 +395,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> !huge_pte_none_mostly(huge_ptep_get(dst_pte))) {
> err = -EEXIST;
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - i_mmap_unlock_read(mapping);
> goto out_unlock;
> }
>
> @@ -409,7 +403,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> wp_copy);
>
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> - i_mmap_unlock_read(mapping);
>
> cond_resched();
>
>