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

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Message-Id: <20200501154853.bca4cfb7b2558bd43a4942f3@linux-foundation.org>
Date:   Fri, 1 May 2020 15:48:53 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     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,  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 "|"?

Also, the return value documentation says we return EINVAL for migration
entries.  Where's that happening, or is the comment out of date?

> 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.