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

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Date:   Fri, 08 May 2020 07:48:01 +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>,
        willy@...radead.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 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  Where's that happening, or is the comment out of date?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.

I remember that Matthew thought that the swap code should work
reasonably even for garbage PTE.

Best Regards,
Huang, Ying

>>Best Regards,
>>Huang, Ying