Re: [PATCH v8 2/2] mm: remove zap_page_range and change callers to use zap_vma_range

[Peter Xu <peterx@redhat.com>]

Hi, Nadav,

On Thu, Nov 10, 2022 at 01:09:43PM -0800, Nadav Amit wrote:
> But, are the callers really able to guarantee that the ranges are all in a
> single VMA? I am not familiar with the users, but how for instance
> tcp_zerocopy_receive() can guarantee that no one did some mprotect() of some
> sorts that caused the original VMA to be split?

Let me try to answer this one for Mike..  We have two callers in tcp
zerocopy code for this function:

tcp_zerocopy_vm_insert_batch_error[2095] zap_page_range(vma, *address, maybe_zap_len);
tcp_zerocopy_receive[2237]     zap_page_range(vma, address, total_bytes_to_map);

Both of them take the mmap lock for read, so firstly mprotect is not
possible.

The 1st call has:

	mmap_read_lock(current->mm);

	vma = vma_lookup(current->mm, address);
	if (!vma || vma->vm_ops != &tcp_vm_ops) {
		mmap_read_unlock(current->mm);
		return -EINVAL;
	}
	vma_len = min_t(unsigned long, zc->length, vma->vm_end - address);
	avail_len = min_t(u32, vma_len, inq);
	total_bytes_to_map = avail_len & ~(PAGE_SIZE - 1);
	if (total_bytes_to_map) {
		if (!(zc->flags & TCP_RECEIVE_ZEROCOPY_FLAG_TLB_CLEAN_HINT))
			zap_page_range(vma, address, total_bytes_to_map);

Here total_bytes_to_map comes from avail_len <--- vma_len, which is a min()
of the rest vma range.  So total_bytes_to_map will never go beyond the vma.

The 2nd call uses maybe_zap_len as len, we need to look two layers of the
callers, but ultimately it's something smaller than total_bytes_to_map we
discussed.  Hopefully it proves 100% safety on tcp zerocopy.

-- 
Peter Xu