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@xxxxxxxxxx>
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@xxxxxxxxxx>
> ---
> 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
>
