Re: [PATCH] mm/shmem: don't release a swapin-error marker as a swap entry

[Baolin Wang]

On 9/20/26 11:50 PM, David Carlier wrote:
> A failed shmem swapin frees the swap slot and leaves a PTE_MARKER_POISONED
> entry in the page cache. On truncate or eviction shmem_free_swap() passes
> that marker to swap_put_entries_direct(), which warns because it is not a
> swap entry. There is nothing left to release either way.
>
> Skip the release for non-swap entries, as every other caller already does.
>
> Reported-by: syzbot+23b25ba3c6bf971f9c57@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=23b25ba3c6bf971f9c57
> Fixes: ac2d3268284b ("mm/swapfile.c: remove the unneeded checking")
> Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
> ---
>   mm/shmem.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index b572c60f2af8..94f2c59c8cfc 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1184,6 +1184,7 @@ static long shmem_free_swap(struct address_space *mapping,
>   			    pgoff_t index, pgoff_t end, void *radswap)
>   {
>   	XA_STATE(xas, &mapping->i_pages, index);
> +	const softleaf_t swp = radix_to_swp_entry(radswap);
>   	unsigned int nr_pages = 0;
>   	pgoff_t base;
>   	void *entry;
> @@ -1200,8 +1201,9 @@ static long shmem_free_swap(struct address_space *mapping,
>   	}
>   	xas_unlock_irq(&xas);
>   
> -	if (nr_pages)
> -		swap_put_entries_direct(radix_to_swp_entry(radswap), nr_pages);
> +	/* A swapin-error marker holds no swap slot, so just drop it. */
> +	if (nr_pages && softleaf_is_swap(swp))
> +		swap_put_entries_direct(swp, nr_pages);
>   
>   	return nr_pages;
>   }

Makes sense to me.

But another issue caught my eye. Since we already call 
shmem_recalc_inode() to decrement 'info->swapped' when handling poisoned 
entries in shmem_set_folio_swapin_error(), shmem_free_swap() still 
returning 'nr_pages' for poisoned entries would cause a second call to 
shmem_recalc_inode(inode, 0, -nr_swaps_freed), thus triggerring 
WARN_ON(i_blocks) in shmem_evict_inode().

I'm not sure if you've observed this warning. IIUC, shmem_free_swap() 
should return 0 for poisoned entries.