linux-kernel - Re: [PATCH 3/3] mm/swapfile.c: count won't be bigger than SWAP_MAP

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Date:   Fri, 8 May 2020 21:19:57 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        willy@...radead.org
Subject: Re: [PATCH 3/3] mm/swapfile.c: count won't be bigger than
 SWAP_MAP_MAX

On Fri, May 08, 2020 at 07:48:01AM +0800, Huang, Ying wrote:
>Wei Yang <richard.weiyang@...il.com> writes:
>
>> On Wed, May 06, 2020 at 04:22:54PM +0800, Huang, Ying wrote:
>>>Wei Yang <richard.weiyang@...il.com> writes:
>>>
>>>> On Fri, May 01, 2020 at 03:48:53PM -0700, Andrew Morton wrote:
>>>>>On Fri,  1 May 2020 01:52:59 +0000 Wei Yang <richard.weiyang@...il.com> wrote:
>>>>>
>>>>>> When the condition is true, there are two possibilities:
>>>>>
>>>>>I'm struggling with this one.
>>>>>
>>>>>>    1. count == SWAP_MAP_BAD
>>>>>>    2. count == (SWAP_MAP_MAX & COUNT_CONTINUED) == SWAP_MAP_SHMEM
>>>>>
>>>>>I'm not sure what 2. is trying to say.  For a start, (SWAP_MAP_MAX &
>>>>>COUNT_CONTINUED) is zero.  I guess it meant "|"?
>>>>
>>>> Oops, you are right. It should be (SWAP_MAP_MAX | COUNT_CONTINUED).
>>>>
>>>> Sorry for the confusion.
>>>>
>>>>>
>>>>>Also, the return value documentation says we return EINVAL for migration
>>>>>entries.  Where's that happening, or is the comment out of date?
>>>>>
>>>>
>>>> Not paid attention to this.
>>>>
>>>> Take look into the code, I don't find a relationship between the swap count
>>>> and migration. Seems we just make a migration entry but not duplicate it.  
>>>> If my understanding is correct.
>>>
>>>Per my understanding, one functionality of the error path is to catch
>>>the behavior that shouldn't happen at all.  For example, if
>>>__swap_duplicate() is called for the migration entry because of some
>>>race condition.
>>>
>>
>> If __swap_duplicate() run for a migration entry, it returns since
>> get_swap_entry() couldn't find a swap_info_struct. So the return value is
>> -EINVAL.
>>
>> While when this situation would happen? And the race condition you mean is?
>
>Sorry for confusing.  I don't mean there are some known race conditions
>in current kernel that will trigger the error code path.  I mean we may
>use the error path to identify some race conditions in the future.
>

Yep, NP.

For the code itself, do you have some comment?

>I remember that Matthew thought that the swap code should work
>reasonably even for garbage PTE.
>
>Best Regards,
>Huang, Ying
>
>>>Best Regards,
>>>Huang, Ying

-- 
Wei Yang
Help you, Help me