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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 11/9/26 20:32, Zi Yan wrote:
> __split_folio_to_order() calls prep_compound_page() on each new head page
> to create a compound page after clear_compound_head(). prep_compound_page()
> uses non atomic __SetPageHead(). It is mostly OK, since the folio is locked
> and frozen and no one else is supposed to modify the folio's flags.
> 
> However, code calling migration_entry_wait() can reach the folio by PFN,
> then access its flags. It is because softleaf_entry_wait_on_locked() used
> by migration_entry_wait() sets PG_waiters concurrently without taking a
> reference and the frozen refcount during split cannot block it. Meanwhile,
> __SetPageHead() does a plain read and write, so PG_waiters atomically set
> by softleaf_entry_wait_on_locked() will be lost if the set happens after
> __SetPageHead()'s read and before its write.
> 
> One example code interleaving:
> 
> CPU0: __split_folio_to_order(new_order>0)  CPU1: do_swap_page()
> 
> 1. clear_compound_head(new_head)
>    new_head->compound_info = 0
>    (page_folio(new_head) == new_head now)
> 
>                                            2. migration_entry_wait()
>                                              softleaf_to_folio(entry)
>                                              page_folio() -> new_head
>                                              (not the old head any more)
> 
> 3. prep_compound_page(new_head, new_order)
>      __SetPageHead(new_head)
>        tmp = new_head->flags.f  <-- load; PG_waiters == 0 in tmp
> 
>                                            4. folio_set_waiters(new_head)
>                                              set_bit(PG_waiters, &flags.f)
>                                              enqueue on waitqueue, sleep
> 
> 5.     new_head->flags.f = tmp | (1UL << PG_head)
>        <-- store; overwrites step 4, PG_waiters == 0 again
> 
> Later, folio_unlock(new_folio) sees no PG_waiters, skips folio_wake_bit(),
> and CPU1 can remain blocked indefinitely.
> 
> Fix it by setting PG_head along with other flags in new_folio->flags before
> smp_wmb() and clear_compound_head(). The remaining compound page metadata
> is set after clear_compound_head(). In that way, after
> clear_compound_head() makes new_folio visible, nothing overwrites the
> PG_waiters from softleaf_entry_wait_on_locked() by modifying
> new_folio->flags non-atomically.
> 
> While at it, use data_race() to suppress the KCSAN issue reported by Ye
> Liu's patch (the second Closes) on reading ->flags during a folio split.
> 
> Suggested-by: Kiryl Shutsemau <kas@kernel.org>
> Reported-by: David Hildenbrand <david@kernel.org>
> Closes: https://lore.kernel.org/all/DMHE8BEEER1H.2BIY52NXATPIA@nvidia.com/
> Reported-by: Ye Liu <ye.liu@linux.dev>
> Closes: https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@linux.dev/
> Fixes: c010d47f107f6 ("mm: thp: split huge page to any lower order pages")
> Cc: stable@vger.kernel.org
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
> The issue was raised by David when reviewing Ye Liu's "mm/huge_memory: use
> READ_ONCE() when cloning folio flags during split" [1]. This patch also
> supersedes Ye Liu's patch.
> 
> [1] https://lore.kernel.org/all/20261029-data_race_huge_mem-v1-1-112e331291c1@linux.dev/
> ---
> Changes in v3:
> - set PG_head before smp_wmb() like Kiryl suggested instead of
>   prep_compound_page_atomic().
> - used data_race() to suppress KCSAN reports while at it.
> - dropped Lorenzo's ack, since the patch is changed.
> - Link to v2: https://patch.msgid.link/20261106-fix-split_folio_to_order-race-v2-1-1b59c67b8af3@nvidia.com
> 
> Changes in v2:
> - renamed __prep_compound_head_and_tail() to __prep_compound_page().
> - added comments to prep_compound_page() and __prep_compound_page().
> - Link to v1: https://patch.msgid.link/20261029-fix-split_folio_to_order-race-v1-1-a483c7451b0d@nvidia.com
> ---
>  mm/huge_memory.c | 32 ++++++++++++++++++++++++++++----
>  mm/internal.h    |  2 ++
>  mm/page_alloc.c  | 17 +++++++++++++++--
>  3 files changed, 45 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bd44d58e86845..99b48d7c988db 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3714,6 +3714,7 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  		 * Don't pass it around before clear_compound_head().
>  		 */
>  		struct folio *new_folio = (struct folio *)new_head;
> +		memdesc_flags_t new_flags;
>  
>  		VM_BUG_ON_PAGE(atomic_read(&new_folio->_mapcount) != -1, new_head);
>  
> @@ -3730,8 +3731,16 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  		 * unreferenced sub-pages of an anonymous THP: we can simply drop
>  		 * PG_anon_exclusive (-> PG_mappedtodisk) for these here.
>  		 */
> -		new_folio->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> -		new_folio->flags.f |= (folio->flags.f &
> +		/*
> +		 * ->flags are stable since the folio is locked and frozen.
> +		 * data_race() is used because reference-less writers,
> +		 * 1. PG_waiters from softleaf_entry_wait_on_locked() and

Hm? I thought that case would be handled by your patch? How can they still write
to the page?

> +		 * 2. PG_hwpoison from memory_failure(),
> +		 * can still write to it.

We lose PG_hwpoison left and right when racing with non-atomic updates. It needs
a fundamental fix. Is this really something we should take care of in this series?

> +		 */
> +		new_flags.f = data_race(new_folio->flags.f) & ~PAGE_FLAGS_CHECK_AT_PREP;
> +		/* See above for data_race() use */
> +		new_flags.f |= data_race(folio->flags.f) &
>  				((1L << PG_referenced) |
>  				 (1L << PG_swapbacked) |
>  				 (1L << PG_swapcache) |
> @@ -3749,7 +3758,18 @@ static void __split_folio_to_order(struct folio *folio, int old_order,
>  #endif
>  				 (1L << PG_dirty) |
>  				 (1L << PG_dropbehind) |
> -				 LRU_GEN_MASK | LRU_REFS_MASK));
> +				 LRU_GEN_MASK | LRU_REFS_MASK);

-- 
Cheers,

David