On 28.09.24 10:34, David Hildenbrand wrote:
On 28.09.24 06:55, Matthew Wilcox wrote:
On Tue, Aug 27, 2024 at 07:47:24PM +0800, Kefeng Wang wrote:
Directly use a folio for HugeTLB and THP when calculate the next pfn, then
remove unused head variable.
I just noticed this got merged. You're going to hit BUG_ON with it.
- if (PageHuge(page)) {
- pfn = page_to_pfn(head) + compound_nr(head) - 1;
- isolate_hugetlb(folio, &source);
- continue;
- } else if (PageTransHuge(page))
- pfn = page_to_pfn(head) + thp_nr_pages(page) - 1;
+ /*
+ * No reference or lock is held on the folio, so it might
+ * be modified concurrently (e.g. split). As such,
+ * folio_nr_pages() may read garbage. This is fine as the outer
+ * loop will revisit the split folio later.
+ */
+ if (folio_test_large(folio)) {
But it's not fine. Look at the implementation of folio_test_large():
static inline bool folio_test_large(const struct folio *folio)
{
return folio_test_head(folio);
}
That's going to be provided by:
#define FOLIO_TEST_FLAG(name, page) \
static __always_inline bool folio_test_##name(const struct folio *folio) \
{ return test_bit(PG_##name, const_folio_flags(folio, page)); }
and here's the BUG:
static const unsigned long *const_folio_flags(const struct folio *folio,
unsigned n)
{
const struct page *page = &folio->page;
VM_BUG_ON_PGFLAGS(PageTail(page), page);
VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
return &page[n].flags;
}
(this page can be transformed from a head page to a tail page because,
as the comment notes, we don't hold a reference.
Please back this out.
Should we generalize the approach in dump_folio() to locally copy a
folio, so we can safely perform checks before deciding whether we want
to try grabbing a reference on the real folio (if it's still a folio :) )?
Oh, and I forgot: isn't the existing code already racy?
PageTransHuge() -> VM_BUG_ON_PAGE(PageTail(page), page);
--
Cheers,
David / dhildenb
