Re: [PATCH v2 7/8] mm: refactor __mmap_region()

[Vlastimil Babka <vbabka@suse.cz>]

On 10/23/24 22:38, Lorenzo Stoakes wrote:
> We have seen bugs and resource leaks arise from the complexity of the
> __mmap_region() function. This, and the generally deeply fragile error
> handling logic and complexity which makes understanding the function
> difficult make it highly desirable to refactor it into something readable.
> 
> Achieve this by separating the function into smaller logical parts which
> are easier to understand and follow, and which importantly very
> significantly simplify the error handling.
> 
> Note that we now call vms_abort_munmap_vmas() in more error paths than we
> used to, however in cases where no abort need occur, vms->nr_pages will be
> equal to zero and we simply exit this function without doing more than we
> would have done previously.
> 
> Importantly, the invocation of the driver mmap hook via mmap_file() now has
> very simple and obvious handling (this was previously the most problematic
> part of the mmap() operation).
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Some nits:

> +static int __mmap_prepare(struct mmap_state *map, struct vma_merge_struct *vmg,
> +	struct list_head *uf)
>  {
> -	struct mm_struct *mm = current->mm;
> -	struct vm_area_struct *vma = NULL;
> -	pgoff_t pglen = PHYS_PFN(len);
> -	unsigned long charged = 0;
> -	struct vma_munmap_struct vms;
> -	struct ma_state mas_detach;
> -	struct maple_tree mt_detach;
> -	unsigned long end = addr + len;
>  	int error;
> -	VMA_ITERATOR(vmi, mm, addr);
> -	VMG_STATE(vmg, mm, &vmi, addr, end, vm_flags, pgoff);
> -
> -	vmg.file = file;
> -	/* Find the first overlapping VMA */
> -	vma = vma_find(&vmi, end);
> -	init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
> -	if (vma) {
> -		mt_init_flags(&mt_detach, vmi.mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> -		mt_on_stack(mt_detach);
> -		mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
> +	struct vma_iterator *vmi = map->vmi;
> +	struct vma_munmap_struct *vms = &map->vms;
> +
> +	/* Find the first overlapping VMA and initialise unmap state. */
> +	vms->vma = vma_find(vmi, map->end);
> +	init_vma_munmap(vms, vmi, vms->vma, map->addr, map->end, uf,
> +			/* unlock = */ false);
> +
> +	/* OK, we have overlapping VMAs - prepare to unmap them. */
> +	if (vms->vma) {
> +		mt_init_flags(&map->mt_detach,
> +			      vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> +		mt_on_stack(map->mt_detach);
> +		mas_init(&map->mas_detach, &map->mt_detach, /* addr = */ 0);
>  		/* Prepare to unmap any existing mapping in the area */
> -		error = vms_gather_munmap_vmas(&vms, &mas_detach);
> -		if (error)
> -			goto gather_failed;
> +		error = vms_gather_munmap_vmas(vms, &map->mas_detach);
> +		if (error) {
> +			/* On error VMAs will already have been reattached. */
> +			vms->nr_pages = 0;
> +			return error;
> +		}
>  
> -		vmg.next = vms.next;
> -		vmg.prev = vms.prev;
> -		vma = NULL;
> +		map->next = vms->next;
> +		map->prev = vms->prev;
>  	} else {
> -		vmg.next = vma_iter_next_rewind(&vmi, &vmg.prev);
> +		map->next = vma_iter_next_rewind(vmi, &map->prev);
>  	}
>  
> +	/* Set up vmg for merge attempt. */
> +	vmg->file = map->file;
> +	vmg->prev = map->prev;
> +	vmg->next = map->next;

It seems arbitrary to do this here. vmg->flags are set in __mmap_region().
Why not all of this? We wouldn't need to pass vmg to __mmap_prepare() at
all? Do any of the values chenge between here and returning and vmg needs to
capture them as they are now? AFAICS not.

> +
>  	/* Check against address space limit. */
> -	if (!may_expand_vm(mm, vm_flags, pglen - vms.nr_pages)) {
> -		error = -ENOMEM;
> -		goto abort_munmap;
> -	}
> +	if (!may_expand_vm(map->mm, map->flags, map->pglen - vms->nr_pages))
> +		return -ENOMEM;
>  
> -	/*
> -	 * Private writable mapping: check memory availability
> -	 */
> -	if (accountable_mapping(file, vm_flags)) {
> -		charged = pglen;
> -		charged -= vms.nr_accounted;
> -		if (charged) {
> -			error = security_vm_enough_memory_mm(mm, charged);
> +	/* Private writable mapping: check memory availability. */
> +	if (accountable_mapping(map->file, map->flags)) {
> +		map->charged = map->pglen;
> +		map->charged -= vms->nr_accounted;
> +		if (map->charged) {
> +			error = security_vm_enough_memory_mm(map->mm, map->charged);
>  			if (error)
> -				goto abort_munmap;
> +				return error;
>  		}
>  
> -		vms.nr_accounted = 0;
> -		vm_flags |= VM_ACCOUNT;
> -		vmg.flags = vm_flags;
> +		vms->nr_accounted = 0;
> +		map->flags |= VM_ACCOUNT;
>  	}
>  
>  	/*
> -	 * clear PTEs while the vma is still in the tree so that rmap
> +	 * Clear PTEs while the vma is still in the tree so that rmap
>  	 * cannot race with the freeing later in the truncate scenario.
>  	 * This is also needed for mmap_file(), which is why vm_ops
>  	 * close function is called.
>  	 */
> -	vms_clean_up_area(&vms, &mas_detach);
> -	vma = vma_merge_new_range(&vmg);
> -	if (vma)
> -		goto expanded;
> +	vms_clean_up_area(vms, &map->mas_detach);
> +
> +	return 0;
> +}
> +

<snip>

> +static int __mmap_new_vma(struct mmap_state *map, struct vma_merge_struct *vmg,
> +			  struct vm_area_struct **vmap)
> +{
> +	struct vma_iterator *vmi = map->vmi;
> +	int error = 0;
> +	bool merged = false;
> +	struct vm_area_struct *vma;
> +
>  	/*
>  	 * Determine the object being mapped and call the appropriate
>  	 * specific mapper. the address has already been validated, but
>  	 * not unmapped, but the maps are removed from the list.
>  	 */
> -	vma = vm_area_alloc(mm);
> -	if (!vma) {
> -		error = -ENOMEM;
> -		goto unacct_error;
> -	}
> +	vma = vm_area_alloc(map->mm);
> +	if (!vma)
> +		return -ENOMEM;
>  
> -	vma_iter_config(&vmi, addr, end);
> -	vma_set_range(vma, addr, end, pgoff);
> -	vm_flags_init(vma, vm_flags);
> -	vma->vm_page_prot = vm_get_page_prot(vm_flags);
> +	vma_iter_config(vmi, map->addr, map->end);
> +	vma_set_range(vma, map->addr, map->end, map->pgoff);
> +	vm_flags_init(vma, map->flags);
> +	vma->vm_page_prot = vm_get_page_prot(map->flags);
>  
> -	if (vma_iter_prealloc(&vmi, vma)) {
> +	if (vma_iter_prealloc(vmi, vma)) {
>  		error = -ENOMEM;
>  		goto free_vma;
>  	}
>  
> -	if (file) {
> -		vma->vm_file = get_file(file);
> -		error = mmap_file(file, vma);
> -		if (error)
> -			goto unmap_and_free_file_vma;
> -
> -		/* Drivers cannot alter the address of the VMA. */
> -		WARN_ON_ONCE(addr != vma->vm_start);
> -		/*
> -		 * Drivers should not permit writability when previously it was
> -		 * disallowed.
> -		 */
> -		VM_WARN_ON_ONCE(vm_flags != vma->vm_flags &&
> -				!(vm_flags & VM_MAYWRITE) &&
> -				(vma->vm_flags & VM_MAYWRITE));
> -
> -		vma_iter_config(&vmi, addr, end);
> -		/*
> -		 * If vm_flags changed after mmap_file(), we should try merge
> -		 * vma again as we may succeed this time.
> -		 */
> -		if (unlikely(vm_flags != vma->vm_flags && vmg.prev)) {
> -			struct vm_area_struct *merge;
> -
> -			vmg.flags = vma->vm_flags;
> -			/* If this fails, state is reset ready for a reattempt. */
> -			merge = vma_merge_new_range(&vmg);
> -
> -			if (merge) {
> -				/*
> -				 * ->mmap() can change vma->vm_file and fput
> -				 * the original file. So fput the vma->vm_file
> -				 * here or we would add an extra fput for file
> -				 * and cause general protection fault
> -				 * ultimately.
> -				 */
> -				fput(vma->vm_file);
> -				vm_area_free(vma);
> -				vma = merge;
> -				/* Update vm_flags to pick up the change. */
> -				vm_flags = vma->vm_flags;
> -				goto file_expanded;
> -			}
> -			vma_iter_config(&vmi, addr, end);
> -		}
> -
> -		vm_flags = vma->vm_flags;
> -	} else if (vm_flags & VM_SHARED) {
> +	if (map->file)
> +		error = __mmap_new_file_vma(map, vmg, &vma, &merged);
> +	else if (map->flags & VM_SHARED)
>  		error = shmem_zero_setup(vma);
> -		if (error)
> -			goto free_iter_vma;
> -	} else {
> +	else
>  		vma_set_anonymous(vma);
> -	}
> +
> +	if (error)
> +		goto free_iter_vma;
> +
> +	if (merged)
> +		goto file_expanded;
>  
>  #ifdef CONFIG_SPARC64
>  	/* TODO: Fix SPARC ADI! */
> -	WARN_ON_ONCE(!arch_validate_flags(vm_flags));
> +	WARN_ON_ONCE(!arch_validate_flags(map->flags));
>  #endif
>  
>  	/* Lock the VMA since it is modified after insertion into VMA tree */
>  	vma_start_write(vma);
> -	vma_iter_store(&vmi, vma);
> -	mm->map_count++;
> +	vma_iter_store(vmi, vma);
> +	map->mm->map_count++;
>  	vma_link_file(vma);
>  
>  	/*
>  	 * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
>  	 * call covers the non-merge case.
>  	 */
> -	khugepaged_enter_vma(vma, vma->vm_flags);
> +	khugepaged_enter_vma(vma, map->flags);
>  
>  file_expanded:
> -	file = vma->vm_file;
>  	ksm_add_vma(vma);

I'm wondering if this could go to the "non file expanded" region above. If
we expanded a vma, it was either in ksm and stays in ksm, or it was not
eligible and that couldn't have changed by expanding?

> -expanded:
> +	*vmap = vma;
> +	return 0;
> +
> +free_iter_vma:
> +	vma_iter_free(vmi);
> +free_vma:
> +	vm_area_free(vma);
> +	return error;
> +}
> +