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

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Message-ID: <20200507222014.6s5szrt6zy2b6ybo@master>
Date:   Thu, 7 May 2020 22:20:14 +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
Subject: Re: [PATCH 3/3] mm/swapfile.c: count won't be bigger than
 SWAP_MAP_MAX

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  Where's that happening, or is the comment out of date?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?

>Best Regards,
>Huang, Ying

-- 
Wei Yang
Help you, Help me