Re: [PATCH v3] mm/huge_memory: prevent losing PG_waiters in __split_folio_to_order()

[Zi Yan]

On 10 Nov 2026, at 8:48, Kiryl Shutsemau wrote:

> On Tue, Nov 10, 2026 at 12:31:21PM +0100, David Hildenbrand (Arm) wrote:
>>> This is V3 of the patch. Instead of using prep_compound_page_atomic(),
>>> PG_head is written before clear_compound_page(), after which new_head
>>> is visible to the reference-less writers.
>>
>> I'm confused, how is it then valid to have PG_waiters set concurrently while we
>> read new_folio->flags.f?
>
> It is not. The comment names the wrong word: PG_waiters can be set
> concurrently on the old head, not on new_folio.

Yes, thank you for the explanation. I confused myself when writing
the comment. At this code place, before clear_compound_head(),
reference-less accessors can only see the old folio.

>
> CPU1 and CPU2 below are two faults on a migration entry that points at
> new_head. Both take this path:
>
>   do_swap_page()
>     migration_entry_wait()
>       softleaf_entry_wait_on_locked()
>         softleaf_to_folio()
>           page_folio()           <-- old head or new_head?
>         folio_set_waiters()
>           set_bit(PG_waiters)    <-- on that folio's flags
>
> Which one page_folio() returns depends on clear_compound_head():
>
> CPU0: __split_folio_to_order()  CPU1: fault           CPU2: fault
>
>                                 1. page_folio()
>                                    -> old head
> 2. flags  = new_folio->flags.f
>    flags |= folio->flags.f
>                                 3. set_bit(PG_waiters)
>                                    -> folio->flags.f
> 4. new_folio->flags.f =
>            flags | PG_head
>    smp_wmb()
> 5. clear_compound_head()
>                                                       6. page_folio()
>                                                          -> new_head
>                                                       7. set_bit(PG_waiters)
>                                                          -> new_folio->flags.f
>
> 2 and 3 race on the old head's flags word.
>
> That is Ye Liu's KCSAN report, and it is benign: PG_waiters is not in
> the cloned mask. new_folio->flags.f gets PG_waiters only at 7, after
> CPU0's last plain write to it at 4.
>
> So only the folio->flags.f read needs data_race().
>
> The new_folio->flags.f read needs nothing; the READ_ONCE() I had there
> in my sketch was not needed.

Yes and I should have checked it more carefully, but my focus was mostly
on whether PG_head could be set before smp_wmb() and clear_compound_head().

I will clean this up and send v4. Sorry for the confusion.

Best Regards,
Yan, Zi