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

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Date:   Wed, 25 May 2022 09:42:51 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     HORIGUCHI NAOYA (堀口 直也) 
        <naoya.horiguchi@....com>, "hughd@...gle.com" <hughd@...gle.com>,
        "willy@...radead.org" <willy@...radead.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "neilb@...e.de" <neilb@...e.de>,
        "apopple@...dia.com" <apopple@...dia.com>,
        "david@...hat.com" <david@...hat.com>,
        "surenb@...gle.com" <surenb@...gle.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "rcampbell@...dia.com" <rcampbell@...dia.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 4/5] mm/shmem: fix infinite loop when swap in shmem
 error at swapoff time

On 2022/5/25 6:10, Andrew Morton wrote:
> On Sat, 21 May 2022 17:34:28 +0800 Miaohe Lin <linmiaohe@...wei.com> 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@....com>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.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).
>>
>> Hi Naoya,
>> I reproduce the issue in the linux-next-next-20220520 version. And I found even if
>> I *do not inject the swapin error*, the deadloop still occurs. After investigating the
>> code for a long while, I found the root cause:
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index d55dd972023a..6d23ed4d23cb 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1182,7 +1182,7 @@ static int shmem_find_swap_entries(struct address_space *mapping,
>>                 if (swp_type(entry) != type)
>>                         continue;
>>
>> -               indices[ret] = xas.xa_index;
>> +               indices[ret++] = xas.xa_index;
>>                 if (!folio_batch_add(fbatch, folio))
>>                         break;
>>
>> The origin code does not increment the ret value when a folio is found. I will send a patch
>> to fix this next week. Thanks! :)
> 
> So I'm thinking that with Hugh's "mm/shmem: fix shmem folio swapoff
> hang", this patchset is now looking OK, so
> 
> mm-swapfile-unuse_pte-can-map-random-data-if-swap-read-fails.patch
> mm-swapfile-fix-lost-swap-bits-in-unuse_pte.patch
> mm-madvise-free-hwpoison-and-swapin-error-entry-in-madvise_free_pte_range.patch
> mm-shmem-fix-infinite-loop-when-swap-in-shmem-error-at-swapoff-time.patch
> mm-filter-out-swapin-error-entry-in-shmem-mapping.patch
> #
> 
> are OK for merging into mainline?

I think so. But it might be better to have Acked-by or Reviewed-by tag for [PATCH v4 4/5] first. :)

Thanks!

> .
>