Re: [PATCH 4/8] mm/memory-failure: Convert shake_page() to shake_folio()

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 06, 2024 at 05:31:19PM +0800, Miaohe Lin wrote:
> > -void shake_page(struct page *p)
> > +void shake_folio(struct folio *folio)
> 
> It might not be a good idea to convert shake_page() into shake_folio(). shake_page() can
> be called without holding page refcnt. So the below race could happen:
> 
>  hwpoison_inject
>   folio = page_folio(p); --> assume p is 4k page [1]
>   shake_folio(folio)
>    folio_test_slab(folio)
>     test_bit(PG_slab, folio_flags(folio, FOLIO_PF_NO_TAIL) [2]
>      VM_BUG_ON_PGFLAGS(PageTail(page), page) in folio_flags
> 
> Between [1] and [2], page can become PageTail of a THP. So the VM_BUG_ON_PGFLAGS will trigger.
> Or am I miss something?

No, you're not missing anything.  This race can happen.  However, I've
removed the VM_BUG_ON for folio_test_slab() with "mm: free up PG_slab".
Now it goes through:

static inline bool PageSlab(const struct page *page)
{
        return folio_test_slab(page_folio(page));
}

static __always_inline bool folio_test_##fname(const struct folio *folio)\
{                                                                       \
        return folio_test_type(folio, PG_##lname);                      \
}                                                                       \

#define folio_test_type(folio, flag)                                    \
        ((folio->page.page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)

which has no assertion that the folio is not a tail page.  Maybe it
should, but until then we'll silently get the wrong result ;-)