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

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Message-ID: <87k11pv5ep.fsf@yhuang-dev.intel.com>
Date:   Wed, 06 May 2020 16:22:54 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     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

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  Where's that happening, or is the comment out of date?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

Best Regards,
Huang, Ying