Re: [PATCH v10 12/25] mm: cache some VMA fields in the vm_fault structure

[Minchan Kim <minchan@kernel.org>]

On Tue, Apr 17, 2018 at 04:33:18PM +0200, Laurent Dufour wrote:
> When handling speculative page fault, the vma->vm_flags and
> vma->vm_page_prot fields are read once the page table lock is released. So
> there is no more guarantee that these fields would not change in our back.
> They will be saved in the vm_fault structure before the VMA is checked for
> changes.

Sorry. I cannot understand.
If it is changed under us, what happens? If it's critical, why cannot we
check with seqcounter?
Clearly, I'm not understanding the logic here. However, it's a global
change without CONFIG_SPF so I want to be more careful.
It would be better to describe why we need to sanpshot those values
into vm_fault rather than preventing the race.

Thanks.

> 
> This patch also set the fields in hugetlb_no_page() and
> __collapse_huge_page_swapin even if it is not need for the callee.
> 
> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
> ---
>  include/linux/mm.h | 10 ++++++++--
>  mm/huge_memory.c   |  6 +++---
>  mm/hugetlb.c       |  2 ++
>  mm/khugepaged.c    |  2 ++
>  mm/memory.c        | 50 ++++++++++++++++++++++++++------------------------
>  mm/migrate.c       |  2 +-
>  6 files changed, 42 insertions(+), 30 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f6edd15563bc..c65205c8c558 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -367,6 +367,12 @@ struct vm_fault {
>  					 * page table to avoid allocation from
>  					 * atomic context.
>  					 */
> +	/*
> +	 * These entries are required when handling speculative page fault.
> +	 * This way the page handling is done using consistent field values.
> +	 */
> +	unsigned long vma_flags;
> +	pgprot_t vma_page_prot;
>  };
>  
>  /* page entry size for vm->huge_fault() */
> @@ -687,9 +693,9 @@ void free_compound_page(struct page *page);
>   * pte_mkwrite.  But get_user_pages can cause write faults for mappings
>   * that do not have writing enabled, when used by access_process_vm.
>   */
> -static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> +static inline pte_t maybe_mkwrite(pte_t pte, unsigned long vma_flags)
>  {
> -	if (likely(vma->vm_flags & VM_WRITE))
> +	if (likely(vma_flags & VM_WRITE))
>  		pte = pte_mkwrite(pte);
>  	return pte;
>  }
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index a3a1815f8e11..da2afda67e68 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1194,8 +1194,8 @@ static int do_huge_pmd_wp_page_fallback(struct vm_fault *vmf, pmd_t orig_pmd,
>  
>  	for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
>  		pte_t entry;
> -		entry = mk_pte(pages[i], vma->vm_page_prot);
> -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		entry = mk_pte(pages[i], vmf->vma_page_prot);
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vmf->vma_flags);
>  		memcg = (void *)page_private(pages[i]);
>  		set_page_private(pages[i], 0);
>  		page_add_new_anon_rmap(pages[i], vmf->vma, haddr, false);
> @@ -2168,7 +2168,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  				entry = pte_swp_mksoft_dirty(entry);
>  		} else {
>  			entry = mk_pte(page + i, READ_ONCE(vma->vm_page_prot));
> -			entry = maybe_mkwrite(entry, vma);
> +			entry = maybe_mkwrite(entry, vma->vm_flags);
>  			if (!write)
>  				entry = pte_wrprotect(entry);
>  			if (!young)
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 218679138255..774864153407 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -3718,6 +3718,8 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
>  				.vma = vma,
>  				.address = address,
>  				.flags = flags,
> +				.vma_flags = vma->vm_flags,
> +				.vma_page_prot = vma->vm_page_prot,
>  				/*
>  				 * Hard to debug if it ends up being
>  				 * used by a callee that assumes
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 0b28af4b950d..2b02a9f9589e 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -887,6 +887,8 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
>  		.flags = FAULT_FLAG_ALLOW_RETRY,
>  		.pmd = pmd,
>  		.pgoff = linear_page_index(vma, address),
> +		.vma_flags = vma->vm_flags,
> +		.vma_page_prot = vma->vm_page_prot,
>  	};
>  
>  	/* we only decide to swapin, if there is enough young ptes */
> diff --git a/mm/memory.c b/mm/memory.c
> index f76f5027d251..2fb9920e06a5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1826,7 +1826,7 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  out_mkwrite:
>  	if (mkwrite) {
>  		entry = pte_mkyoung(entry);
> -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vma->vm_flags);
>  	}
>  
>  	set_pte_at(mm, addr, pte, entry);
> @@ -2472,7 +2472,7 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
>  
>  	flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
>  	entry = pte_mkyoung(vmf->orig_pte);
> -	entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +	entry = maybe_mkwrite(pte_mkdirty(entry), vmf->vma_flags);
>  	if (ptep_set_access_flags(vma, vmf->address, vmf->pte, entry, 1))
>  		update_mmu_cache(vma, vmf->address, vmf->pte);
>  	pte_unmap_unlock(vmf->pte, vmf->ptl);
> @@ -2548,8 +2548,8 @@ static int wp_page_copy(struct vm_fault *vmf)
>  			inc_mm_counter_fast(mm, MM_ANONPAGES);
>  		}
>  		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
> -		entry = mk_pte(new_page, vma->vm_page_prot);
> -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		entry = mk_pte(new_page, vmf->vma_page_prot);
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vmf->vma_flags);
>  		/*
>  		 * Clear the pte entry and flush it first, before updating the
>  		 * pte with the new entry. This will avoid a race condition
> @@ -2614,7 +2614,7 @@ static int wp_page_copy(struct vm_fault *vmf)
>  		 * Don't let another task, with possibly unlocked vma,
>  		 * keep the mlocked page.
>  		 */
> -		if (page_copied && (vma->vm_flags & VM_LOCKED)) {
> +		if (page_copied && (vmf->vma_flags & VM_LOCKED)) {
>  			lock_page(old_page);	/* LRU manipulation */
>  			if (PageMlocked(old_page))
>  				munlock_vma_page(old_page);
> @@ -2650,7 +2650,7 @@ static int wp_page_copy(struct vm_fault *vmf)
>   */
>  int finish_mkwrite_fault(struct vm_fault *vmf)
>  {
> -	WARN_ON_ONCE(!(vmf->vma->vm_flags & VM_SHARED));
> +	WARN_ON_ONCE(!(vmf->vma_flags & VM_SHARED));
>  	if (!pte_map_lock(vmf))
>  		return VM_FAULT_RETRY;
>  	/*
> @@ -2752,7 +2752,7 @@ static int do_wp_page(struct vm_fault *vmf)
>  		 * We should not cow pages in a shared writeable mapping.
>  		 * Just mark the pages writable and/or call ops->pfn_mkwrite.
>  		 */
> -		if ((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
> +		if ((vmf->vma_flags & (VM_WRITE|VM_SHARED)) ==
>  				     (VM_WRITE|VM_SHARED))
>  			return wp_pfn_shared(vmf);
>  
> @@ -2799,7 +2799,7 @@ static int do_wp_page(struct vm_fault *vmf)
>  			return VM_FAULT_WRITE;
>  		}
>  		unlock_page(vmf->page);
> -	} else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
> +	} else if (unlikely((vmf->vma_flags & (VM_WRITE|VM_SHARED)) ==
>  					(VM_WRITE|VM_SHARED))) {
>  		return wp_page_shared(vmf);
>  	}
> @@ -3078,9 +3078,9 @@ int do_swap_page(struct vm_fault *vmf)
>  
>  	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
>  	dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
> -	pte = mk_pte(page, vma->vm_page_prot);
> +	pte = mk_pte(page, vmf->vma_page_prot);
>  	if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page, NULL)) {
> -		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> +		pte = maybe_mkwrite(pte_mkdirty(pte), vmf->vma_flags);
>  		vmf->flags &= ~FAULT_FLAG_WRITE;
>  		ret |= VM_FAULT_WRITE;
>  		exclusive = RMAP_EXCLUSIVE;
> @@ -3105,7 +3105,7 @@ int do_swap_page(struct vm_fault *vmf)
>  
>  	swap_free(entry);
>  	if (mem_cgroup_swap_full(page) ||
> -	    (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
> +	    (vmf->vma_flags & VM_LOCKED) || PageMlocked(page))
>  		try_to_free_swap(page);
>  	unlock_page(page);
>  	if (page != swapcache && swapcache) {
> @@ -3163,7 +3163,7 @@ static int do_anonymous_page(struct vm_fault *vmf)
>  	pte_t entry;
>  
>  	/* File mapping without ->vm_ops ? */
> -	if (vma->vm_flags & VM_SHARED)
> +	if (vmf->vma_flags & VM_SHARED)
>  		return VM_FAULT_SIGBUS;
>  
>  	/*
> @@ -3187,7 +3187,7 @@ static int do_anonymous_page(struct vm_fault *vmf)
>  	if (!(vmf->flags & FAULT_FLAG_WRITE) &&
>  			!mm_forbids_zeropage(vma->vm_mm)) {
>  		entry = pte_mkspecial(pfn_pte(my_zero_pfn(vmf->address),
> -						vma->vm_page_prot));
> +						vmf->vma_page_prot));
>  		if (!pte_map_lock(vmf))
>  			return VM_FAULT_RETRY;
>  		if (!pte_none(*vmf->pte))
> @@ -3220,8 +3220,8 @@ static int do_anonymous_page(struct vm_fault *vmf)
>  	 */
>  	__SetPageUptodate(page);
>  
> -	entry = mk_pte(page, vma->vm_page_prot);
> -	if (vma->vm_flags & VM_WRITE)
> +	entry = mk_pte(page, vmf->vma_page_prot);
> +	if (vmf->vma_flags & VM_WRITE)
>  		entry = pte_mkwrite(pte_mkdirty(entry));
>  
>  	if (!pte_map_lock(vmf)) {
> @@ -3418,7 +3418,7 @@ static int do_set_pmd(struct vm_fault *vmf, struct page *page)
>  	for (i = 0; i < HPAGE_PMD_NR; i++)
>  		flush_icache_page(vma, page + i);
>  
> -	entry = mk_huge_pmd(page, vma->vm_page_prot);
> +	entry = mk_huge_pmd(page, vmf->vma_page_prot);
>  	if (write)
>  		entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
>  
> @@ -3492,11 +3492,11 @@ int alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>  		return VM_FAULT_NOPAGE;
>  
>  	flush_icache_page(vma, page);
> -	entry = mk_pte(page, vma->vm_page_prot);
> +	entry = mk_pte(page, vmf->vma_page_prot);
>  	if (write)
> -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> +		entry = maybe_mkwrite(pte_mkdirty(entry), vmf->vma_flags);
>  	/* copy-on-write page */
> -	if (write && !(vma->vm_flags & VM_SHARED)) {
> +	if (write && !(vmf->vma_flags & VM_SHARED)) {
>  		inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
>  		page_add_new_anon_rmap(page, vma, vmf->address, false);
>  		mem_cgroup_commit_charge(page, memcg, false, false);
> @@ -3535,7 +3535,7 @@ int finish_fault(struct vm_fault *vmf)
>  
>  	/* Did we COW the page? */
>  	if ((vmf->flags & FAULT_FLAG_WRITE) &&
> -	    !(vmf->vma->vm_flags & VM_SHARED))
> +	    !(vmf->vma_flags & VM_SHARED))
>  		page = vmf->cow_page;
>  	else
>  		page = vmf->page;
> @@ -3789,7 +3789,7 @@ static int do_fault(struct vm_fault *vmf)
>  		ret = VM_FAULT_SIGBUS;
>  	else if (!(vmf->flags & FAULT_FLAG_WRITE))
>  		ret = do_read_fault(vmf);
> -	else if (!(vma->vm_flags & VM_SHARED))
> +	else if (!(vmf->vma_flags & VM_SHARED))
>  		ret = do_cow_fault(vmf);
>  	else
>  		ret = do_shared_fault(vmf);
> @@ -3846,7 +3846,7 @@ static int do_numa_page(struct vm_fault *vmf)
>  	 * accessible ptes, some can allow access by kernel mode.
>  	 */
>  	pte = ptep_modify_prot_start(vma->vm_mm, vmf->address, vmf->pte);
> -	pte = pte_modify(pte, vma->vm_page_prot);
> +	pte = pte_modify(pte, vmf->vma_page_prot);
>  	pte = pte_mkyoung(pte);
>  	if (was_writable)
>  		pte = pte_mkwrite(pte);
> @@ -3880,7 +3880,7 @@ static int do_numa_page(struct vm_fault *vmf)
>  	 * Flag if the page is shared between multiple address spaces. This
>  	 * is later used when determining whether to group tasks together
>  	 */
> -	if (page_mapcount(page) > 1 && (vma->vm_flags & VM_SHARED))
> +	if (page_mapcount(page) > 1 && (vmf->vma_flags & VM_SHARED))
>  		flags |= TNF_SHARED;
>  
>  	last_cpupid = page_cpupid_last(page);
> @@ -3925,7 +3925,7 @@ static inline int wp_huge_pmd(struct vm_fault *vmf, pmd_t orig_pmd)
>  		return vmf->vma->vm_ops->huge_fault(vmf, PE_SIZE_PMD);
>  
>  	/* COW handled on pte level: split pmd */
> -	VM_BUG_ON_VMA(vmf->vma->vm_flags & VM_SHARED, vmf->vma);
> +	VM_BUG_ON_VMA(vmf->vma_flags & VM_SHARED, vmf->vma);
>  	__split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
>  
>  	return VM_FAULT_FALLBACK;
> @@ -4072,6 +4072,8 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  		.flags = flags,
>  		.pgoff = linear_page_index(vma, address),
>  		.gfp_mask = __get_fault_gfp_mask(vma),
> +		.vma_flags = vma->vm_flags,
> +		.vma_page_prot = vma->vm_page_prot,
>  	};
>  	unsigned int dirty = flags & FAULT_FLAG_WRITE;
>  	struct mm_struct *mm = vma->vm_mm;
> diff --git a/mm/migrate.c b/mm/migrate.c
> index bb6367d70a3e..44d7007cfc1c 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -240,7 +240,7 @@ static bool remove_migration_pte(struct page *page, struct vm_area_struct *vma,
>  		 */
>  		entry = pte_to_swp_entry(*pvmw.pte);
>  		if (is_write_migration_entry(entry))
> -			pte = maybe_mkwrite(pte, vma);
> +			pte = maybe_mkwrite(pte, vma->vm_flags);
>  
>  		if (unlikely(is_zone_device_page(new))) {
>  			if (is_device_private_page(new)) {
> -- 
> 2.7.4
>