Re: [PATCH 2/9] mm/page_alloc: clear ->private of deferred free pages

[Zi Yan]

On 27 Oct 2026, at 11:46, Vlastimil Babka (SUSE) wrote:

> From: Josef Bacik <josef@xxxxxxxxxxxxxx>
>
> free_pages_nolock() and free_frozen_pages_nolock() put the page on
> zone->trylock_free_pages if they get neither the pcp lock nor zone->lock.
> add_page_to_zone_llist() keeps the order of the page in page->private.
> The next free_one_page() which does get zone->lock passes these pages to
> split_large_buddy() with ->private still set.
>
> __free_one_page() writes ->private of the head of the resulting free page
> only. If the deferred page is merged with its lower buddy, it becomes a
> tail of that free page and keeps the order in ->private.
>
> All other free pages which are not the head of a buddy have a ->private
> of zero, and since commit 819bfbd47e49 ("mm/page_alloc: remove
> set_page_private() in prep_compound_tail()") the allocator relies on it.
> When the stale page is allocated again as a tail of a larger page:
>
>  - prep_compound_tail() hits VM_WARN_ON_ONCE(tail->private);
>  - with the page checks enabled, __free_pages_prepare() reports
>    "BUG: Bad page state", "nonzero private", and leaks the whole page;
>  - without them the value is seen by whoever uses that page next, e.g. as
>    folio->private of a folio which is split off a large folio.
>
> Only pages of an order above zero are affected. Those can be freed
> through _nolock() by the stack depot (order 2) and by SLUB (a new slab
> which cannot be added to the partial list by kmalloc_nolock()).
>
> Clear ->private when the page is taken off the list.
>
> Fixes: 8c57b687e833 ("mm, bpf: Introduce free_pages_nolock()")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> ---
>  mm/page_alloc.c | 2 ++
>  1 file changed, 2 insertions(+)
>
Makes sense.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi