Re: [PATCH v6 3/3] mm: implement page refcount locking via dedicated bit

[Matthew Wilcox <willy@infradead.org>]

On Sat, Sep 12, 2026 at 10:50:10PM +0300, Ilya Gladyshev wrote:
> This patch reallocates the refcount value range:
> 
> (1) refcount < 0 means dead refcount (uninit / frozen)
> (2) refcount = 0 allowed only as a temporary state (see below)
> (3) refcount > 0 is a regular reference count
> 
> In other words, refcount is now split into "dead bit" and a 31-bit
> counter.

It seems to me that refcount overflow is now a problem.

We can deliberately increase the refcount on a page by stuffing it into
a pipe.  Over and over again.  See merge commit 6b3a70773630 and the four
commits on that branch: f958d7b528b1 88b1a17dfc3e 8fde12ca79af 15fab63e1e57

Unfortunately, I think the discussion that led to those commits was
conducted off-list because security.  I wish we had a way to declassify
thse emails after the fact.

> +/* Most significant bit in page refcount */
> +#define PAGEREF_FROZEN_BIT BIT(31)
> +
> +/* Page reference counter can be in 3 logical states,
> + * which are described below with their value representation
> + *        state              |         value
> + * (1)  safe with  owners    |   1...INT_MAX
> + * (2)  safe with no owners  |         0
> + * (3)  frozen               |  INT_MIN....-1

I think we need four states.  The first two are the same.

(3) frozen: 			0xc000'0000 - 0xffff'ffff
(4) temporarily overflown:	0x8000'0000 - 0xbfff'ffff

(we don't really need that much space for temporary overflow; we could
have something like 0x8100'0000 as the boundary if that works out better)

>  static inline bool __page_count_is_frozen(int count)
>  {
> -	return count == 0;
> +	return count & PAGEREF_FROZEN_BIT;

This probably becomes '(unsigned)count >> 30 == 3'.  If we choose a
different boundary then something like (unsigned)count >> 24 >= 0x81.

>  static inline int page_ref_count(const struct page *page)
>  {
> -	return atomic_read(&page->_refcount);
> +	int val = atomic_read(&page->_refcount);
> +
> +	if (unlikely(val & PAGEREF_FROZEN_BIT))
> +		return 0;

... use __page_count_is_frozen() here?

and you need to adjust folio_ref_zero_or_close_to_overflow().
try_get_page() can stay as it is.

(Thanks to Pedro for asking me annoying questions about mapcount
overflow which prompted me to look at this again)