Re: [PATCH v1 16/36] fs: hugetlbfs: cleanup folio in adjust_range_hwpoison()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Aug 28, 2025 at 12:01:20AM +0200, David Hildenbrand wrote:
> Let's cleanup and simplify the function a bit.

Ah I guess you separated this out from the previous patch? :)

I feel like it might be worth talking about the implementation here in the
commit message as it took me a while to figure this out.

>
> Signed-off-by: David Hildenbrand <david@redhat.com>


This original implementation is SO GROSS.

God this hurts my mind

		n = min(bytes, (size_t)PAGE_SIZE - offset);

So either it'll be remaining bytes in page or we're only spanning one page first
time round

Then we

		res += n;
		bytes -= n;

So bytes comes to end of page if spanning multiple

Then offset if spanning multiple pages will be PAGE_SIZE -offset + offset (!!!)
therefore PAGE_SIZE And we move to the next page and reset offset to 0:

		offset += n;
		if (offset == PAGE_SIZE) {
			page = nth_page(page, 1);
			offset = 0;
		}

Then from then on n = min(bytes, PAGE_SIZE) (!!!!!!)

So res = remaining safe bytes in first page + num other pages OR bytes if we
don't span more than 1.

Lord above.

Also semantics of 'if bytes == 0, then check first page anyway' which you do
capture.

OK think I have convinced myself this is right, so hopefully no deeply subtle
off-by-one issues here :P

Anyway, LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
>  fs/hugetlbfs/inode.c | 33 +++++++++++----------------------
>  1 file changed, 11 insertions(+), 22 deletions(-)
>
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index c5a46d10afaa0..6ca1f6b45c1e5 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -198,31 +198,20 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
>  static size_t adjust_range_hwpoison(struct folio *folio, size_t offset,
>  		size_t bytes)
>  {
> -	struct page *page;
> -	size_t n = 0;
> -	size_t res = 0;
> -
> -	/* First page to start the loop. */
> -	page = folio_page(folio, offset / PAGE_SIZE);
> -	offset %= PAGE_SIZE;
> -	while (1) {
> -		if (is_raw_hwpoison_page_in_hugepage(page))
> -			break;
> +	struct page *page = folio_page(folio, offset / PAGE_SIZE);
> +	size_t safe_bytes;
> +
> +	if (is_raw_hwpoison_page_in_hugepage(page))
> +		return 0;
> +	/* Safe to read the remaining bytes in this page. */
> +	safe_bytes = PAGE_SIZE - (offset % PAGE_SIZE);
> +	page++;
>
> -		/* Safe to read n bytes without touching HWPOISON subpage. */
> -		n = min(bytes, (size_t)PAGE_SIZE - offset);
> -		res += n;
> -		bytes -= n;
> -		if (!bytes || !n)
> +	for (; safe_bytes < bytes; safe_bytes += PAGE_SIZE, page++)

OK this is quite subtle - so if safe_bytes == bytes, this means we've confirmed
that all requested bytes are safe.

So offset=0, bytes = 4096 would fail this (as safe_bytes == 4096).

Maybe worth putting something like:

	/*
	 * Now we check page-by-page in the folio to see if any bytes we don't
	 * yet know to be safe are contained within posioned pages or not.
	 */

Above the loop. Or something like this.

> +		if (is_raw_hwpoison_page_in_hugepage(page))
>  			break;
> -		offset += n;
> -		if (offset == PAGE_SIZE) {
> -			page++;
> -			offset = 0;
> -		}
> -	}
>
> -	return res;
> +	return min(safe_bytes, bytes);

Yeah given above analysis this seems correct.

You must have torn your hair out over this :)

>  }
>
>  /*
> --
> 2.50.1
>