Re: [PATCH 2/4] mm/memory-failure.c: fix wrong user reference report

[Miaohe Lin <linmiaohe@huawei.com>]

On 2022/3/8 4:14, Yang Shi wrote:
> On Mon, Mar 7, 2022 at 3:26 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>>
>> On 2022/3/4 16:27, HORIGUCHI NAOYA(堀口 直也) wrote:
>>> On Mon, Feb 28, 2022 at 10:02:43PM +0800, Miaohe Lin wrote:
>>>> The dirty swapcache page is still residing in the swap cache after it's
>>>> hwpoisoned. So there is always one extra refcount for swap cache.
>>>
>>> The diff seems fine at a glance, but let me have a few question to
>>> understand the issue more.
>>>
>>> - Is the behavior described above the effect of recent change on shmem where
>>>   dirty pagecache is pinned on hwpoison (commit a76054266661 ("mm: shmem:
>>>   don't truncate page if memory failure happens"). Or the older kernels
>>>   behave as the same?
>>>
>>> - Is the behavior true for normal anonymous pages (not shmem pages)?
>>>
>>
>> The behavior described above is aimed at swapcache not pagecache. So it should be
>> irrelevant with the recent change on shmem.
>>
>> What I try to fix here is that me_swapcache_dirty holds one extra pin via SwapCache
>> regardless of the return value of delete_from_lru_cache. We should try to report more
>> accurate extra refcount for debugging purpose.
> 
> I think you misunderstood the code. The delete_from_lru_cache()
> returning 0 means the page was on LRU and isolated from LRU
> successfully now. Returning -EIO means the page was not on LRU, so it
> should have at least an extra pin on it.
> 
> So MF_DELAYED means there is no other pin other than hwpoison and
> swapcache which is expected, MF_FAILED means there might be extra
> pins.
> 
> The has_extra_refcount() raised error then there is *unexpected* refcount.

Many thanks for your explanation. It seems you're right. If page is held on
the lru_pvecs when we try to do delete_from_lru_cache, and after that it's
drained to the lru list( so its refcnt might be 2 now). Then we might have
the following complain if extra_pins is always true:
	"Memory failure: ... still referenced by 0 users\n"

But it seems the origin code can not report the correct reason too because
if we retry, page can be delete_from_lru_cache and we can succeed now.

Anyway, many thanks for pointing this out.

> 
>>
>>> I'm trying to test hwpoison hitting the dirty swapcache, but it seems that
>>> in my testing memory_faliure() fails with "hwpoison: unhandlable page"
>>
>> Maybe memory_faliure is racing with page reclaim where page is isolated?
>>
>>> warning at get_any_page().  So I'm still not sure that me_pagecache_dirty()
>>> fixes any visible problem.
>>
>> IIUC, me_pagecache_dirty can't do much except for the corresponding address_space supporting
>> error_remove_page which can truncate the dirty pagecache page. But this may cause silent data
>> loss. It's better to keep the page stay in the pagecache until the file is truncated, hole
>> punched or removed as commit a76054266661 pointed out.
>>
>> Thanks.
>>
>>>> Thanks,
>>> Naoya Horiguchi
>>>
>>>>
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>> ---
>>>>  mm/memory-failure.c | 6 +-----
>>>>  1 file changed, 1 insertion(+), 5 deletions(-)
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index 0d7c58340a98..5f9503573263 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -984,7 +984,6 @@ static int me_pagecache_dirty(struct page_state *ps, struct page *p)
>>>>  static int me_swapcache_dirty(struct page_state *ps, struct page *p)
>>>>  {
>>>>      int ret;
>>>> -    bool extra_pins = false;
>>>>
>>>>      ClearPageDirty(p);
>>>>      /* Trigger EIO in shmem: */
>>>> @@ -993,10 +992,7 @@ static int me_swapcache_dirty(struct page_state *ps, struct page *p)
>>>>      ret = delete_from_lru_cache(p) ? MF_FAILED : MF_DELAYED;
>>>>      unlock_page(p);
>>>>
>>>> -    if (ret == MF_DELAYED)
>>>> -            extra_pins = true;
>>>> -
>>>> -    if (has_extra_refcount(ps, p, extra_pins))
>>>> +    if (has_extra_refcount(ps, p, true))
>>>>              ret = MF_FAILED;
>>>>
>>>>      return ret;
>>>> --
>>>> 2.23.0
>>
>>
> .
>