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From:   Kairui Song <ryncsn@gmail.com>
Date:   Thu, 23 Nov 2023 19:13:27 +0800
Message-ID: <CAMgjq7Ba9inFHCqoZSNY_gFUv=UEVEC8FvG+V7-7Qpv027tEmQ@mail.gmail.com>
Subject: Re: [PATCH 16/24] mm/swap: reduce scope of get_swap_device in swapin path
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
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Huang, Ying <ying.huang@intel.com> =E4=BA=8E2023=E5=B9=B411=E6=9C=8822=E6=
=97=A5=E5=91=A8=E4=B8=89 08:38=E5=86=99=E9=81=93=EF=BC=9A
>
> Kairui Song <ryncsn@gmail.com> writes:
>
> > From: Kairui Song <kasong@tencent.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.