Re: [PATCH v3] mm/page_alloc: clear page->private in free_pages_prepare()

[Baolin Wang]

On 2/10/26 10:12 AM, Zi Yan wrote:
> On 9 Feb 2026, at 20:20, Baolin Wang wrote:
>
> > On 2/10/26 3:42 AM, Zi Yan wrote:
> > > On 9 Feb 2026, at 14:39, David Hildenbrand (Arm) wrote:
> > >
> > > > On 2/9/26 18:44, Zi Yan wrote:
> > > > > On 9 Feb 2026, at 12:36, David Hildenbrand (Arm) wrote:
> > > > >
> > > > > > On 2/9/26 17:33, Zi Yan wrote:
> > > > > > > I agree. Silently fixing non zero ->private just moves the work/responsibility
> > > > > > > from users to core mm. They could do better. :)
> > > > > > >
> > > > > > > We can have a patch or multiple patches to fix users do not zero ->private
> > > > > > > when freeing a page and add the patch below.
> > > > > >
> > > > > > Do we know roughly which ones don't zero it out?
> > > > >
> > > > > So far based on [1], I found:
> > > > >
> > > > > 1. shmem_swapin_folio() in mm/shmem.c does not zero ->swap.val (overlapping
> > > > > with private);
> >
> > After Kairui’s series [1], the shmem part looks good to me. As we no longer skip the swapcache now, we shouldn’t clear the ->swap.val of a swapcache folio if failed to swap-in.
>
> What do you mean by "after Kairui's series[1]"? Can you elaborate a little bit more?

Sure. This patch [2] in Kairui's series will never skip the swapcache, 
which means the shmem folio we’re trying to swap-in must be in the 
swapcache.

[2] 
https://lore.kernel.org/all/20251219195751.61328-1-ryncsn@xxxxxxxxx/T/#me242d9f77d2caa126124afd5a7731113e8f0346e

> For the diff below, does the "folio_put(folio)" have different outcomes based on
> skip_swapcache? Only if skip_swapcache is true, "folio_put(folio)" frees the folio?

Please check the latest mm-stable branch. The skip_swapcache related 
logic has been removed by Kairui’s series [1].

> diff --git a/mm/shmem.c b/mm/shmem.c
> index ec6c01378e9d..546e193ef993 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2437,8 +2437,10 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>   failed_nolock:
>          if (skip_swapcache)
>                  swapcache_clear(si, folio->swap, folio_nr_pages(folio));
> -       if (folio)
> +       if (folio) {
> +               folio->swap.val = 0;
>                  folio_put(folio);
> +       }
>          put_swap_device(si);
>
>          return error;

Without Kairui's series, this change is incorrect. Yes, only if 
skip_swapcache is true, the "folio_put(folio)" frees the folio. 
Otherwise the folio is in the swapcache, and we will not free it.

> > [1]https://lore.kernel.org/all/20251219195751.61328-1-ryncsn@xxxxxxxxx/T/#mcba8a32e1021dc28ce1e824c9d042dca316a30d7