Re: [PATCH v2 2/6] ntfs: Remove use of __folio_index in handle_bounds_compressed_page()

[Lance Yang <lance.yang@linux.dev>]

On Tue, Jun 09, 2026 at 04:55:03PM +0100, Usama Arif wrote:
[...]
>> 
>> Are you saying my transformation is non-equivalent (and I have
>> introduced a bug), or are you noting that this is a pre-existing bug?
>> 
>> If the latter, is the error that the else branch exists, or did the
>> author intend to write a different test?
>
>The transformation is non-equivalent.

Right. Let me try to make one case easier to see.

Assume:

  PAGE_SIZE             = 4096
  page->__folio_index   = 2
  pos                   = 8192
  initialized_size      = 9000
  i_size                = 12000

Before the patch, the relevant code was:

	if ((page->__folio_index >= (initialized_size >> PAGE_SHIFT)) &&
			(initialized_size < i_size)) {
[...]

		if (((s64)page->__folio_index << PAGE_SHIFT) >= initialized_size) {
			clear_page(kp);
			return;
		}
		kp_ofs = initialized_size & ~PAGE_MASK;
		memset(kp + kp_ofs, 0, PAGE_SIZE - kp_ofs);
	}

For this page, the outer check was true:

  page->__folio_index >= (initialized_size >> PAGE_SHIFT)
  2 >= (9000 >> 12)
  2 >= 2

and initialized_size < i_size was also true:

  initialized_size < i_size
  9000 < 12000

but the inner check was false:

  ((s64)page->__folio_index << PAGE_SHIFT) >= initialized_size
  (2 << 12) >= 9000
  8192 >= 9000

So the old code reached the tail-zeroing case:

  kp_ofs = 9000 & ~PAGE_MASK = 808

and zeroed page offset 808..4095, i.e. file offset 9000..12287.

After this patch, the outer check became:

loff_t pos = page_offset(page);

if ((pos >= initialized_size) && (initialized_size < i_size)) 

For the same page:

  pos >= initialized_size
  8192 >= 9000

so the first half of the outer check is false, and the block is skipped
entirely.

Cheers, Lance