Re: [PATCH v6 2/3] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Muhammad Usama Anjum]

Hello Andrei,

Thank you for reviewing.

On 11/10/22 4:54 AM, Andrei Vagin wrote:
[...]
>> +static int add_to_out(bool sd, bool file, bool pres, bool swap, struct pagemap_scan_private *p,
>> +		      unsigned long addr, unsigned int len)
>> +{
>> +	unsigned long bitmap, cur = sd | file << 1 | pres << 2 | swap << 3;
> 
> Should we define contants for each of these bits?
I think I can define a macro to hide this dirty bit shifting in the function.

> 
>> +	bool cpy = true;
>> +
>> +	if (p->required_mask)
>> +		cpy = ((p->required_mask & cur) == p->required_mask);
>> +	if (cpy && p->anyof_mask)
>> +		cpy = (p->anyof_mask & cur);
>> +	if (cpy && p->excluded_mask)
>> +		cpy = !(p->excluded_mask & cur);
>> +
>> +	bitmap = cur & p->return_mask;
>> +
>> +	if (cpy && bitmap) {
>> +		if ((p->vec_index) && (p->vec[p->vec_index - 1].bitmap == bitmap) &&
>> +		    (p->vec[p->vec_index - 1].start + p->vec[p->vec_index - 1].len * PAGE_SIZE ==
>> +		     addr)) {
> 
> I think it is better to define a variable for p->vec_index - 1.
Will do in the next revision.

> nit: len can be in bytes rather than pages.
We are considering memory in the page units. The memory given to this IOCTL
must have PAGE_SIZE alignment. Oterwise we error out (picked this from
mincore()).

>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long addr,
>> +				  unsigned long end, struct mm_walk *walk)
>> +{
>> +	struct pagemap_scan_private *p = walk->private;
>> +	struct vm_area_struct *vma = walk->vma;
>> +	unsigned int len;
>> +	spinlock_t *ptl;
>> +	int ret = 0;
>> +	pte_t *pte;
>> +	bool dirty_vma = (p->flags & PAGEMAP_NO_REUSED_REGIONS) ?
>> +			 (false) : (vma->vm_flags & VM_SOFTDIRTY);
>> +
>> +	if ((walk->vma->vm_end < addr) || (p->max_pages && p->found_pages == p->max_pages))
>> +		return 0;
>> +
>> +	end = min(end, walk->vma->vm_end);
>> +
>> +	ptl = pmd_trans_huge_lock(pmd, vma);
>> +	if (ptl) {
>> +		if (dirty_vma || check_soft_dirty_pmd(vma, addr, pmd, false)) {
>> +			/*
>> +			 * Break huge page into small pages if operation needs to be performed is
>> +			 * on a portion of the huge page or the return buffer cannot store complete
>> +			 * data.
>> +			 */
>> +			if ((IS_CLEAR_OP(p) && (end - addr < HPAGE_SIZE))) {
>> +				spin_unlock(ptl);
>> +				split_huge_pmd(vma, pmd, addr);
>> +				goto process_smaller_pages;
>> +			}
>> +
>> +			if (IS_GET_OP(p)) {
>> +				len = (end - addr)/PAGE_SIZE;
>> +				if (p->max_pages && p->found_pages + len > p->max_pages)
>> +					len = p->max_pages - p->found_pages;
>> +
>> +				ret = add_to_out(dirty_vma ||
>> +						 check_soft_dirty_pmd(vma, addr, pmd, false),
> 
> can we reuse a return code of the previous call of check_soft_dirty_pmd?
Yes, will do.

> 
>> +						 vma->vm_file, pmd_present(*pmd), is_swap_pmd(*pmd),
>> +						 p, addr, len);
>> +			}
>> +			if (!ret && IS_CLEAR_OP(p))
>> +				check_soft_dirty_pmd(vma, addr, pmd, true);
> 
> should we return a error in this case? We need to be sure that:
> * we stop waking page tables after this point.
I'll update the implementation to return error. It immediately terminates
the walk as well.
> * return this error to the user-space if we are not able to add anything
>   in the vector.
I'm not returning error to userspace if we found no page matching the
masks. The total number of filled page_region are returned from the IOCTL.
If IOCTL returns 0, it means no page found has found, but the IOCTL
executed successfully.

[...]
>> +static long do_pagemap_sd_cmd(struct mm_struct *mm, struct pagemap_scan_arg *arg)
>> +{
>> +	struct mmu_notifier_range range;
>> +	unsigned long __user start, end;
>> +	struct pagemap_scan_private p;
>> +	int ret;
>> +
>> +	start = (unsigned long)untagged_addr(arg->start);
>> +	if ((!IS_ALIGNED(start, PAGE_SIZE)) || (!access_ok((void __user *)start, arg->len)))
>> +		return -EINVAL;
>> +
>> +	if (IS_GET_OP(arg) &&
>> +	    ((arg->vec_len == 0) || (!access_ok((struct page_region *)arg->vec, arg->vec_len))))
>> +		return -ENOMEM;
>> +
>> +#ifndef CONFIG_MEM_SOFT_DIRTY
>> +	if (IS_SD_OP(arg) || (arg->required_mask & PAGE_IS_SOFTDIRTY) ||
>> +	    (arg->anyof_mask & PAGE_IS_SOFTDIRTY))
>> +		return -EINVAL;
>> +#endif
>> +
>> +	if ((arg->flags & ~PAGEMAP_SD_FLAGS) || (arg->required_mask & ~PAGEMAP_OP_MASK) ||
>> +	    (arg->anyof_mask & ~PAGEMAP_OP_MASK) || (arg->excluded_mask & ~PAGEMAP_OP_MASK) ||
>> +	    (arg->return_mask & ~PAGEMAP_OP_MASK))
>> +		return -EINVAL;
>> +
>> +	if ((!arg->required_mask && !arg->anyof_mask && !arg->excluded_mask) || !arg->return_mask)
>> +		return -EINVAL;
>> +
>> +	if (IS_SD_OP(arg) && ((arg->required_mask & PAGEMAP_NONSD_OP_MASK) ||
>> +	     (arg->anyof_mask & PAGEMAP_NONSD_OP_MASK)))
>> +		return -EINVAL;
>> +
>> +	end = start + arg->len;
>> +	p.max_pages = arg->max_pages;
>> +	p.found_pages = 0;
>> +	p.flags = arg->flags;
>> +	p.required_mask = arg->required_mask;
>> +	p.anyof_mask = arg->anyof_mask;
>> +	p.excluded_mask = arg->excluded_mask;
>> +	p.return_mask = arg->return_mask;
>> +	p.vec_index = 0;
>> +	p.vec_len = arg->vec_len;
>> +
>> +	if (IS_GET_OP(arg)) {
>> +		p.vec = vzalloc(arg->vec_len * sizeof(struct page_region));
> 
> I think we need to set a reasonable limit for vec_len to avoid large
> allocations on the kernel. We can consider to use kmalloc or kvmalloc
> here.
I'll update to kvzalloc which uses vmalloc if kmalloc fails. It'll use
kmalloc for smaller allocations. Thanks for suggesting it. But it'll not
limit the memory allocation.

> 
> Thanks,
> Andrei

-- 
BR,
Muhammad Usama Anjum