Re: [PATCH v4 6/7] mm: introduce __vm_flags_mod and use it in untrack_pfn

[Mike Rapoport <rppt@kernel.org>]

On Thu, Jan 26, 2023 at 11:37:51AM -0800, Suren Baghdasaryan wrote:
> There are scenarios when vm_flags can be modified without exclusive
> mmap_lock, such as:
> - after VMA was isolated and mmap_lock was downgraded or dropped
> - in exit_mmap when there are no other mm users and locking is unnecessary
> Introduce __vm_flags_mod to avoid assertions when the caller takes
> responsibility for the required locking.
> Pass a hint to untrack_pfn to conditionally use __vm_flags_mod for
> flags modification to avoid assertion.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Acked-by: Michal Hocko <mhocko@suse.com>

Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>

> ---
>  arch/x86/mm/pat/memtype.c | 10 +++++++---
>  include/linux/mm.h        | 14 ++++++++++++--
>  include/linux/pgtable.h   |  5 +++--
>  mm/memory.c               | 13 +++++++------
>  mm/memremap.c             |  4 ++--
>  mm/mmap.c                 | 16 ++++++++++------
>  6 files changed, 41 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index 6ca51b1aa5d9..691bf8934b6f 100644
> --- a/arch/x86/mm/pat/memtype.c
> +++ b/arch/x86/mm/pat/memtype.c
> @@ -1046,7 +1046,7 @@ void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot, pfn_t pfn)
>   * can be for the entire vma (in which case pfn, size are zero).
>   */
>  void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
> -		 unsigned long size)
> +		 unsigned long size, bool mm_wr_locked)
>  {
>  	resource_size_t paddr;
>  	unsigned long prot;
> @@ -1065,8 +1065,12 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
>  		size = vma->vm_end - vma->vm_start;
>  	}
>  	free_pfn_range(paddr, size);
> -	if (vma)
> -		vm_flags_clear(vma, VM_PAT);
> +	if (vma) {
> +		if (mm_wr_locked)
> +			vm_flags_clear(vma, VM_PAT);
> +		else
> +			__vm_flags_mod(vma, 0, VM_PAT);
> +	}
>  }
>  
>  /*
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 3c7fc3ecaece..a00fdeb4492d 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -656,6 +656,16 @@ static inline void vm_flags_clear(struct vm_area_struct *vma,
>  	ACCESS_PRIVATE(vma, __vm_flags) &= ~flags;
>  }
>  
> +/*
> + * Use only if VMA is not part of the VMA tree or has no other users and
> + * therefore needs no locking.
> + */
> +static inline void __vm_flags_mod(struct vm_area_struct *vma,
> +				  vm_flags_t set, vm_flags_t clear)
> +{
> +	vm_flags_init(vma, (vma->vm_flags | set) & ~clear);
> +}
> +
>  /*
>   * Use only when the order of set/clear operations is unimportant, otherwise
>   * use vm_flags_{set|clear} explicitly.
> @@ -664,7 +674,7 @@ static inline void vm_flags_mod(struct vm_area_struct *vma,
>  				vm_flags_t set, vm_flags_t clear)
>  {
>  	mmap_assert_write_locked(vma->vm_mm);
> -	vm_flags_init(vma, (vma->vm_flags | set) & ~clear);
> +	__vm_flags_mod(vma, set, clear);
>  }
>  
>  static inline void vma_set_anonymous(struct vm_area_struct *vma)
> @@ -2102,7 +2112,7 @@ static inline void zap_vma_pages(struct vm_area_struct *vma)
>  }
>  void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
>  		struct vm_area_struct *start_vma, unsigned long start,
> -		unsigned long end);
> +		unsigned long end, bool mm_wr_locked);
>  
>  struct mmu_notifier_range;
>  
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 5fd45454c073..c63cd44777ec 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -1185,7 +1185,8 @@ static inline int track_pfn_copy(struct vm_area_struct *vma)
>   * can be for the entire vma (in which case pfn, size are zero).
>   */
>  static inline void untrack_pfn(struct vm_area_struct *vma,
> -			       unsigned long pfn, unsigned long size)
> +			       unsigned long pfn, unsigned long size,
> +			       bool mm_wr_locked)
>  {
>  }
>  
> @@ -1203,7 +1204,7 @@ extern void track_pfn_insert(struct vm_area_struct *vma, pgprot_t *prot,
>  			     pfn_t pfn);
>  extern int track_pfn_copy(struct vm_area_struct *vma);
>  extern void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
> -			unsigned long size);
> +			unsigned long size, bool mm_wr_locked);
>  extern void untrack_pfn_moved(struct vm_area_struct *vma);
>  #endif
>  
> diff --git a/mm/memory.c b/mm/memory.c
> index a6316cda0e87..7a04a1130ec1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1613,7 +1613,7 @@ void unmap_page_range(struct mmu_gather *tlb,
>  static void unmap_single_vma(struct mmu_gather *tlb,
>  		struct vm_area_struct *vma, unsigned long start_addr,
>  		unsigned long end_addr,
> -		struct zap_details *details)
> +		struct zap_details *details, bool mm_wr_locked)
>  {
>  	unsigned long start = max(vma->vm_start, start_addr);
>  	unsigned long end;
> @@ -1628,7 +1628,7 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>  		uprobe_munmap(vma, start, end);
>  
>  	if (unlikely(vma->vm_flags & VM_PFNMAP))
> -		untrack_pfn(vma, 0, 0);
> +		untrack_pfn(vma, 0, 0, mm_wr_locked);
>  
>  	if (start != end) {
>  		if (unlikely(is_vm_hugetlb_page(vma))) {
> @@ -1675,7 +1675,7 @@ static void unmap_single_vma(struct mmu_gather *tlb,
>   */
>  void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
>  		struct vm_area_struct *vma, unsigned long start_addr,
> -		unsigned long end_addr)
> +		unsigned long end_addr, bool mm_wr_locked)
>  {
>  	struct mmu_notifier_range range;
>  	struct zap_details details = {
> @@ -1689,7 +1689,8 @@ void unmap_vmas(struct mmu_gather *tlb, struct maple_tree *mt,
>  				start_addr, end_addr);
>  	mmu_notifier_invalidate_range_start(&range);
>  	do {
> -		unmap_single_vma(tlb, vma, start_addr, end_addr, &details);
> +		unmap_single_vma(tlb, vma, start_addr, end_addr, &details,
> +				 mm_wr_locked);
>  	} while ((vma = mas_find(&mas, end_addr - 1)) != NULL);
>  	mmu_notifier_invalidate_range_end(&range);
>  }
> @@ -1723,7 +1724,7 @@ void zap_page_range_single(struct vm_area_struct *vma, unsigned long address,
>  	 * unmap 'address-end' not 'range.start-range.end' as range
>  	 * could have been expanded for hugetlb pmd sharing.
>  	 */
> -	unmap_single_vma(&tlb, vma, address, end, details);
> +	unmap_single_vma(&tlb, vma, address, end, details, false);
>  	mmu_notifier_invalidate_range_end(&range);
>  	tlb_finish_mmu(&tlb);
>  }
> @@ -2492,7 +2493,7 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
>  
>  	err = remap_pfn_range_notrack(vma, addr, pfn, size, prot);
>  	if (err)
> -		untrack_pfn(vma, pfn, PAGE_ALIGN(size));
> +		untrack_pfn(vma, pfn, PAGE_ALIGN(size), true);
>  	return err;
>  }
>  EXPORT_SYMBOL(remap_pfn_range);
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 08cbf54fe037..2f88f43d4a01 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -129,7 +129,7 @@ static void pageunmap_range(struct dev_pagemap *pgmap, int range_id)
>  	}
>  	mem_hotplug_done();
>  
> -	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range));
> +	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range), true);
>  	pgmap_array_delete(range);
>  }
>  
> @@ -276,7 +276,7 @@ static int pagemap_range(struct dev_pagemap *pgmap, struct mhp_params *params,
>  	if (!is_private)
>  		kasan_remove_zero_shadow(__va(range->start), range_len(range));
>  err_kasan:
> -	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range));
> +	untrack_pfn(NULL, PHYS_PFN(range->start), range_len(range), true);
>  err_pfn_remap:
>  	pgmap_array_delete(range);
>  	return error;
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 3d9b14d5f933..429e42c8fccc 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -78,7 +78,7 @@ core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
>  static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
>  		struct vm_area_struct *vma, struct vm_area_struct *prev,
>  		struct vm_area_struct *next, unsigned long start,
> -		unsigned long end);
> +		unsigned long end, bool mm_wr_locked);
>  
>  static pgprot_t vm_pgprot_modify(pgprot_t oldprot, unsigned long vm_flags)
>  {
> @@ -2136,14 +2136,14 @@ static inline void remove_mt(struct mm_struct *mm, struct ma_state *mas)
>  static void unmap_region(struct mm_struct *mm, struct maple_tree *mt,
>  		struct vm_area_struct *vma, struct vm_area_struct *prev,
>  		struct vm_area_struct *next,
> -		unsigned long start, unsigned long end)
> +		unsigned long start, unsigned long end, bool mm_wr_locked)
>  {
>  	struct mmu_gather tlb;
>  
>  	lru_add_drain();
>  	tlb_gather_mmu(&tlb, mm);
>  	update_hiwater_rss(mm);
> -	unmap_vmas(&tlb, mt, vma, start, end);
> +	unmap_vmas(&tlb, mt, vma, start, end, mm_wr_locked);
>  	free_pgtables(&tlb, mt, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
>  				 next ? next->vm_start : USER_PGTABLES_CEILING);
>  	tlb_finish_mmu(&tlb);
> @@ -2391,7 +2391,11 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  			mmap_write_downgrade(mm);
>  	}
>  
> -	unmap_region(mm, &mt_detach, vma, prev, next, start, end);
> +	/*
> +	 * We can free page tables without write-locking mmap_lock because VMAs
> +	 * were isolated before we downgraded mmap_lock.
> +	 */
> +	unmap_region(mm, &mt_detach, vma, prev, next, start, end, !downgrade);
>  	/* Statistics and freeing VMAs */
>  	mas_set(&mas_detach, start);
>  	remove_mt(mm, &mas_detach);
> @@ -2704,7 +2708,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  
>  		/* Undo any partial mapping done by a device driver. */
>  		unmap_region(mm, &mm->mm_mt, vma, prev, next, vma->vm_start,
> -			     vma->vm_end);
> +			     vma->vm_end, true);
>  	}
>  	if (file && (vm_flags & VM_SHARED))
>  		mapping_unmap_writable(file->f_mapping);
> @@ -3031,7 +3035,7 @@ void exit_mmap(struct mm_struct *mm)
>  	tlb_gather_mmu_fullmm(&tlb, mm);
>  	/* update_hiwater_rss(mm) here? but nobody should be looking */
>  	/* Use ULONG_MAX here to ensure all VMAs in the mm are unmapped */
> -	unmap_vmas(&tlb, &mm->mm_mt, vma, 0, ULONG_MAX);
> +	unmap_vmas(&tlb, &mm->mm_mt, vma, 0, ULONG_MAX, false);
>  	mmap_read_unlock(mm);
>  
>  	/*
> -- 
> 2.39.1
> 

-- 
Sincerely yours,
Mike.