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

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Date:   Sat, 2 May 2020 13:29:11 +0000
From:   Wei Yang <richard.weiyang@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Wei Yang <richard.weiyang@...il.com>, ying.huang@...el.com,
        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 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  Where's that happening, or is the comment out of date?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.

>> The first case would be filtered by the first if in __swap_duplicate().
>> 
>> And the second case means this swap entry is for shmem. Since we never
>> do another duplication for shmem swap entry. This won't happen neither.
>

-- 
Wei Yang
Help you, Help me