linux-kernel - Re: [PATCH 16/24] mm/swap: reduce scope of get_swap

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Date:   Fri, 24 Nov 2023 08:40:56 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Kairui Song <ryncsn@...il.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/24] mm/swap: reduce scope of get_swap_device in
 swapin path

Kairui Song <ryncsn@...il.com> writes:

> Huang, Ying <ying.huang@...el.com> 于2023年11月22日周三 08:38写道：
>>
>> Kairui Song <ryncsn@...il.com> writes:
>>
>> > From: Kairui Song <kasong@...cent.com>
>> >
>> > Move get_swap_device into swapin_readahead, simplify the code
>> > and prepare for follow up commits.
>>
>> No.  Please don't do this.  Please check the get/put_swap_device() usage
>> rule in the comments of get_swap_device().
>>
>> "
>>  * When we get a swap entry, if there aren't some other ways to
>>  * prevent swapoff, such as the folio in swap cache is locked, page
>>  * table lock is held, etc., the swap entry may become invalid because
>>  * of swapoff.  Then, we need to enclose all swap related functions
>>  * with get_swap_device() and put_swap_device(), unless the swap
>>  * functions call get/put_swap_device() by themselves.
>> "
>>
>> This is to simplify the reasoning about swapoff and swap entry.
>>
>> Why does it bother you?
>
> Hi Ying,
>
> This is trying to reduce LOC, avoid a trivial si read, and make error
> checking logic easier to refactor in later commits.

The race with swapoff isn't considered by many developers usually.  So,
we should use a simple rule as much as possible.  For example, if you
get a swap entry, use get/put_swap_device() to enclose all code that
operate on the swap entry.  This makes code easier to be maintained in
the long run.  Yes.  Sometimes we break the rule a little, but only if
we have enough benefit, such as improving performance in some practical
use cases.

> And besides there is one trivial change I forgot to include in this
> commit, get_swap_device can be put after swap_cache_get_folio in
> swapin_readahead, since swap_cache_get_folio doesn't need to hold the
> swap device, so in cache hit case this get/put_swap_device() can be
> saved.

swapoff is rare operation, it's OK to delay it a little to make the code
easier to be understood.

> The comment also mentioned:
>
> "Then, we need to enclose all swap related functions with
> get_swap_device() and put_swap_device(), unless the swap functions
> call get/put_swap_device() by themselves"
>
> So I think it should be OK to do this.

No.  You should change the code with a good enough reason.  Compared
with complexity it introduced, the benefit isn't enough for me so far.

--
Best Regards,
Huang, Ying