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

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/5/23 7:53, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Fri, May 20, 2022 at 04:17:45PM +0800, Miaohe Lin wrote:
>> On 2022/5/20 14:34, HORIGUCHI NAOYA(堀口 直也) wrote:
>>> 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).
>>
>> I was testing this patch on my 5.10 kernel. I reproduced the problem in my env and
>> fixed it. It seems there might be some critical difference though I checked that by
>> reviewing the code... Sorry. :(
>>
>>>
>>> This patch has the effect to change the return value of shmem_swapin_folio(),
>>> -EIO (without this patch) to -EEXIST (with this patch).
>>
>> In fact, I didn't change the return value from -EIO to -EEXIST:
>>
>> @@ -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)
>>
>>> 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;
>>>         }
>>
>> Yes, this is the simplest and straightforward way to fix the issue. But it has the side effect
>> that user will end up with a permanently mounted swap just because a sector is bad. That might
>> be somewhat unacceptable?
> 
> Ah, you're right, swapoff should return with success instead of with
> failure.  I tried the fix in your another email, and that makes swapoff
> return with success, so your fix looks better than mine.

I reproduced the deadloop issues when swapin error occurs at swapoff time in my linux-next-next-20220520 env,
and I found this patch could solve the issue now with the fix in my another email.

BTW: When I use dm-dust to inject the swapin IO error, I don't see non-uptodate folio when shmem_swapin_folio
and swapoff succeeds. There might be some issues around that module (so I resort to the another way to inject
the swapin error), but the patch itself works anyway. ;)

> 
> Thanks,

Thanks a lot!

> Naoya Horiguchi
>