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@xxxxxxx>
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
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);
