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

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Date:   Thu, 23 Nov 2023 19:13:27 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     "Huang, Ying" <ying.huang@...el.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

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.

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.

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.