Re: [PATCH v2 12/26] userfaultfd: wp: apply _PAGE_UFFD_WP bit

[Mike Rapoport]

On Tue, Feb 12, 2019 at 10:56:18AM +0800, Peter Xu wrote:
> Firstly, introduce two new flags MM_CP_UFFD_WP[_RESOLVE] for
> change_protection() when used with uffd-wp and make sure the two new
> flags are exclusively used.  Then,
> 
>   - For MM_CP_UFFD_WP: apply the _PAGE_UFFD_WP bit and remove _PAGE_RW
>     when a range of memory is write protected by uffd
> 
>   - For MM_CP_UFFD_WP_RESOLVE: remove the _PAGE_UFFD_WP bit and recover
>     _PAGE_RW when write protection is resolved from userspace
> 
> And use this new interface in mwriteprotect_range() to replace the old
> MM_CP_DIRTY_ACCT.
> 
> Do this change for both PTEs and huge PMDs.  Then we can start to
> identify which PTE/PMD is write protected by general (e.g., COW or soft
> dirty tracking), and which is for userfaultfd-wp.
> 
> Since we should keep the _PAGE_UFFD_WP when doing pte_modify(), add it
> into _PAGE_CHG_MASK as well.  Meanwhile, since we have this new bit, we
> can be even more strict when detecting uffd-wp page faults in either
> do_wp_page() or wp_huge_pmd().
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx>

> ---
>  arch/x86/include/asm/pgtable_types.h |  2 +-
>  include/linux/mm.h                   |  5 +++++
>  mm/huge_memory.c                     | 14 +++++++++++++-
>  mm/memory.c                          |  4 ++--
>  mm/mprotect.c                        | 12 ++++++++++++
>  mm/userfaultfd.c                     |  8 ++++++--
>  6 files changed, 39 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 8cebcff91e57..dd9c6295d610 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -133,7 +133,7 @@
>   */
>  #define _PAGE_CHG_MASK	(PTE_PFN_MASK | _PAGE_PCD | _PAGE_PWT |		\
>  			 _PAGE_SPECIAL | _PAGE_ACCESSED | _PAGE_DIRTY |	\
> -			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP)
> +			 _PAGE_SOFT_DIRTY | _PAGE_DEVMAP | _PAGE_UFFD_WP)
>  #define _HPAGE_CHG_MASK (_PAGE_CHG_MASK | _PAGE_PSE)
> 
>  /*
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 9fe3b0066324..f38fbe9c8bc9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1657,6 +1657,11 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
>  #define  MM_CP_DIRTY_ACCT                  (1UL << 0)
>  /* Whether this protection change is for NUMA hints */
>  #define  MM_CP_PROT_NUMA                   (1UL << 1)
> +/* Whether this change is for write protecting */
> +#define  MM_CP_UFFD_WP                     (1UL << 2) /* do wp */
> +#define  MM_CP_UFFD_WP_RESOLVE             (1UL << 3) /* Resolve wp */
> +#define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
> +					    MM_CP_UFFD_WP_RESOLVE)
> 
>  extern unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
>  			      unsigned long end, pgprot_t newprot,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8d65b0f041f9..817335b443c2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1868,6 +1868,8 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  	bool preserve_write;
>  	int ret;
>  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> +	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> +	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> 
>  	ptl = __pmd_trans_huge_lock(pmd, vma);
>  	if (!ptl)
> @@ -1934,6 +1936,13 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>  	entry = pmd_modify(entry, newprot);
>  	if (preserve_write)
>  		entry = pmd_mk_savedwrite(entry);
> +	if (uffd_wp) {
> +		entry = pmd_wrprotect(entry);
> +		entry = pmd_mkuffd_wp(entry);
> +	} else if (uffd_wp_resolve) {
> +		entry = pmd_mkwrite(entry);
> +		entry = pmd_clear_uffd_wp(entry);
> +	}
>  	ret = HPAGE_PMD_NR;
>  	set_pmd_at(mm, addr, pmd, entry);
>  	BUG_ON(vma_is_anonymous(vma) && !preserve_write && pmd_write(entry));
> @@ -2083,7 +2092,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  	struct page *page;
>  	pgtable_t pgtable;
>  	pmd_t old_pmd, _pmd;
> -	bool young, write, soft_dirty, pmd_migration = false;
> +	bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
>  	unsigned long addr;
>  	int i;
> 
> @@ -2165,6 +2174,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  		write = pmd_write(old_pmd);
>  		young = pmd_young(old_pmd);
>  		soft_dirty = pmd_soft_dirty(old_pmd);
> +		uffd_wp = pmd_uffd_wp(old_pmd);
>  	}
>  	VM_BUG_ON_PAGE(!page_count(page), page);
>  	page_ref_add(page, HPAGE_PMD_NR - 1);
> @@ -2198,6 +2208,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  				entry = pte_mkold(entry);
>  			if (soft_dirty)
>  				entry = pte_mksoft_dirty(entry);
> +			if (uffd_wp)
> +				entry = pte_mkuffd_wp(entry);
>  		}
>  		pte = pte_offset_map(&_pmd, addr);
>  		BUG_ON(!pte_none(*pte));
> diff --git a/mm/memory.c b/mm/memory.c
> index 00781c43407b..f8d83ae16eff 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2483,7 +2483,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  {
>  	struct vm_area_struct *vma = vmf->vma;
> 
> -	if (userfaultfd_wp(vma)) {
> +	if (userfaultfd_pte_wp(vma, *vmf->pte)) {
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
>  		return handle_userfault(vmf, VM_UFFD_WP);
>  	}
> @@ -3692,7 +3692,7 @@ static inline vm_fault_t create_huge_pmd(struct vm_fault *vmf)
>  static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
>  {
>  	if (vma_is_anonymous(vmf->vma)) {
> -		if (userfaultfd_wp(vmf->vma))
> +		if (userfaultfd_huge_pmd_wp(vmf->vma, orig_pmd))
>  			return handle_userfault(vmf, VM_UFFD_WP);
>  		return do_huge_pmd_wp_page(vmf, orig_pmd);
>  	}
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index a6ba448c8565..9d4433044c21 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -46,6 +46,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  	int target_node = NUMA_NO_NODE;
>  	bool dirty_accountable = cp_flags & MM_CP_DIRTY_ACCT;
>  	bool prot_numa = cp_flags & MM_CP_PROT_NUMA;
> +	bool uffd_wp = cp_flags & MM_CP_UFFD_WP;
> +	bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE;
> 
>  	/*
>  	 * Can be called with only the mmap_sem for reading by
> @@ -117,6 +119,14 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  			if (preserve_write)
>  				ptent = pte_mk_savedwrite(ptent);
> 
> +			if (uffd_wp) {
> +				ptent = pte_wrprotect(ptent);
> +				ptent = pte_mkuffd_wp(ptent);
> +			} else if (uffd_wp_resolve) {
> +				ptent = pte_mkwrite(ptent);
> +				ptent = pte_clear_uffd_wp(ptent);
> +			}
> +
>  			/* Avoid taking write faults for known dirty pages */
>  			if (dirty_accountable && pte_dirty(ptent) &&
>  					(pte_soft_dirty(ptent) ||
> @@ -301,6 +311,8 @@ unsigned long change_protection(struct vm_area_struct *vma, unsigned long start,
>  {
>  	unsigned long pages;
> 
> +	BUG_ON((cp_flags & MM_CP_UFFD_WP_ALL) == MM_CP_UFFD_WP_ALL);
> +
>  	if (is_vm_hugetlb_page(vma))
>  		pages = hugetlb_change_protection(vma, start, end, newprot);
>  	else
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 73a208c5c1e7..80bcd642911d 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -73,8 +73,12 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm,
>  		goto out_release;
> 
>  	_dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));
> -	if (dst_vma->vm_flags & VM_WRITE && !wp_copy)
> -		_dst_pte = pte_mkwrite(_dst_pte);
> +	if (dst_vma->vm_flags & VM_WRITE) {
> +		if (wp_copy)
> +			_dst_pte = pte_mkuffd_wp(_dst_pte);
> +		else
> +			_dst_pte = pte_mkwrite(_dst_pte);
> +	}
> 
>  	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
>  	if (dst_vma->vm_file) {
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.