Re: [PATCH v3 1/5] mm: memory_hotplug: remove head variable in do_migrate_range()

[David Hildenbrand <david@redhat.com>]

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