Re: [PATCH] mm/page_idle: call folio_test_lru() after folio_get()

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Wed, Jul 22, 2026 at 05:26:42PM +0800, Jiale Yao wrote:
> page_idle_get_folio() speculatively calls folio_test_lru() before
> folio_try_get(). The folio can get freed and reallocated to a tail page
> in the meantime. In that case, VM_BUG_ON_PGFLAGS() in
> const_folio_flags() can be triggered. Remove the speculative call.
>
> Also mark the folio_test_lru() check right after folio_try_get() success
> as no more unlikely.

Slightly strange wording but not sure why you're doing that? It is
generally unlikely a given folio will be !LRU right?

>
> This is a sibling-path bug: damon_get_folio() was copied from this
> function with the same flawed pattern. Commit d6b8b02a27b3
> ("mm/damon/ops-common: call folio_test_lru() after folio_get()") fixed
> damon_get_folio(), but page_idle_get_folio() was left unfixed. KCSAN
> (strict mode) confirms the data race on the folio flags:
>
>   BUG: KCSAN: data-race in ... / percpu_counter_add_batch
>     page_idle_get_folio+0x7a/0x2d0
>     page_idle_bitmap_read+0xc9/0x220
>
> Signed-off-by: Jiale Yao <yaojiale02@163.com>

Yeah generally this seems obviously correct (TM), if we can't be sure a
folio is kept around any other way to the extent we're doing
folio_try_get() we should gate any actual interactions with the folio on
succeeding the get first...!

With the unlikely thing changed, LGTM so:

Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/page_idle.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_idle.c b/mm/page_idle.c
> index 9c67cbac2965..29ee18f0e8ce 100644
> --- a/mm/page_idle.c
> +++ b/mm/page_idle.c
> @@ -40,9 +40,9 @@ static struct folio *page_idle_get_folio(unsigned long pfn)
>  		return NULL;
>
>  	folio = page_folio(page);
> -	if (!folio_test_lru(folio) || !folio_try_get(folio))

I guess this was meant as a racey check...

> +	if (!folio_try_get(folio))
>  		return NULL;
> -	if (unlikely(page_folio(page) != folio || !folio_test_lru(folio))) {
> +	if (unlikely(page_folio(page) != folio) || !folio_test_lru(folio)) {

As above, not sure why you're changing this? Seems unrelated, I'd just keep
it as it was.

>  		folio_put(folio);
>  		folio = NULL;
>  	}
> --
> 2.34.1
>

Cheers, Lorenzo