Re: [PATCH v4 4/5] mm/shmem: fix infinite loop when swap in shmem error at swapoff time

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Thu, May 19, 2022 at 08:50:29PM +0800, Miaohe Lin wrote:
> When swap in shmem error at swapoff time, there would be a infinite loop
> in the while loop in shmem_unuse_inode(). It's because swapin error is
> deliberately ignored now and thus info->swapped will never reach 0. So
> we can't escape the loop in shmem_unuse().
> 
> In order to fix the issue, swapin_error entry is stored in the mapping
> when swapin error occurs. So the swapcache page can be freed and the
> user won't end up with a permanently mounted swap because a sector is
> bad. If the page is accessed later, the user process will be killed
> so that corrupted data is never consumed. On the other hand, if the
> page is never accessed, the user won't even notice it.
> 
> Reported-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>

Hi Miaohe,

Thank you for the update.  I might miss something, but I still see the same
problem (I checked it on mm-everything-2022-05-19-00-03 + this patchset).

This patch has the effect to change the return value of shmem_swapin_folio(),
-EIO (without this patch) to -EEXIST (with this patch).
But shmem_unuse_swap_entries() checks neither, so no change from caller's view point.
Maybe breaking in errors (rather than ENOMEM) in for loop in shmem_unuse_swap_entries()
solves the issue?  I briefly checked with the below change, then swapoff can return
with failure.

@@ -1222,7 +1222,7 @@ static int shmem_unuse_swap_entries(struct inode *inode,
                        folio_put(folio);
                        ret++;
                }
-               if (error == -ENOMEM)
+               if (error < 0)
                        break;
                error = 0;
        }

> ---
>  mm/shmem.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d3c7970e0179..d55dd972023a 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1175,6 +1175,10 @@ static int shmem_find_swap_entries(struct address_space *mapping,
>  			continue;
>  
>  		entry = radix_to_swp_entry(folio);
> +		/*
> +		 * swapin error entries can be found in the mapping. But they're
> +		 * deliberately ignored here as we've done everything we can do.
> +		 */
>  		if (swp_type(entry) != type)
>  			continue;
>  
> @@ -1672,6 +1676,36 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
>  	return error;
>  }
>  
> +static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
> +					 struct folio *folio, swp_entry_t swap)
> +{
> +	struct address_space *mapping = inode->i_mapping;
> +	struct shmem_inode_info *info = SHMEM_I(inode);
> +	swp_entry_t swapin_error;
> +	void *old;
> +
> +	swapin_error = make_swapin_error_entry(&folio->page);
> +	old = xa_cmpxchg_irq(&mapping->i_pages, index,
> +			     swp_to_radix_entry(swap),
> +			     swp_to_radix_entry(swapin_error), 0);
> +	if (old != swp_to_radix_entry(swap))
> +		return;
> +
> +	folio_wait_writeback(folio);
> +	delete_from_swap_cache(&folio->page);
> +	spin_lock_irq(&info->lock);
> +	/*
> +	 * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks won't
> +	 * be 0 when inode is released and thus trigger WARN_ON(inode->i_blocks) in
> +	 * shmem_evict_inode.
> +	 */
> +	info->alloced--;
> +	info->swapped--;
> +	shmem_recalc_inode(inode);
> +	spin_unlock_irq(&info->lock);
> +	swap_free(swap);
> +}
> +
>  /*
>   * Swap in the page pointed to by *pagep.
>   * Caller has to make sure that *pagep contains a valid swapped page.

(off-topic a little) BTW, the comment on shmem_swapin_folio() still mentions
*pagep, but maybe it can be updated to *foliop.

Thanks,
Naoya Horiguchi

> @@ -1695,6 +1729,9 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>  	swap = radix_to_swp_entry(*foliop);
>  	*foliop = NULL;
>  
> +	if (is_swapin_error_entry(swap))
> +		return -EIO;
> +
>  	/* Look it up and read it in.. */
>  	page = lookup_swap_cache(swap, NULL, 0);
>  	if (!page) {
> @@ -1762,6 +1799,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>  failed:
>  	if (!shmem_confirm_swap(mapping, index, swap))
>  		error = -EEXIST;
> +	if (error == -EIO)
> +		shmem_set_folio_swapin_error(inode, index, folio, swap);