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