linux-kernel - Re: [RFC PATCH 0/2] remove SWAP_MAP

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Message-ID: <CAJD7tkbjr14JbbPOadacjbUmKEPyqPwAjikFO1xRCkOkLhN8LA@mail.gmail.com>
Date: Mon, 23 Sep 2024 17:20:58 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: akpm@...ux-foundation.org, hannes@...xchg.org, hughd@...gle.com, 
	shakeel.butt@...ux.dev, ryan.roberts@....com, ying.huang@...el.com, 
	chrisl@...nel.org, david@...hat.com, kasong@...cent.com, willy@...radead.org, 
	viro@...iv.linux.org.uk, baohua@...nel.org, chengming.zhou@...ux.dev, 
	linux-mm@...ck.org, kernel-team@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

On Mon, Sep 23, 2024 at 4:11 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> The SWAP_MAP_SHMEM state was originally introduced in the commit
> aaa468653b4a ("swap_info: note SWAP_MAP_SHMEM"), to quickly determine if a
> swap entry belongs to shmem during swapoff.
>
> However, swapoff has since been rewritten drastically in the commit
> b56a2d8af914 ("mm: rid swapoff of quadratic complexity"). Now
> having swap count == SWAP_MAP_SHMEM value is basically the same as having
> swap count == 1, and swap_shmem_alloc() behaves analogously to
> swap_duplicate()
>
> This RFC proposes the removal of this state and the associated helper to
> simplify the state machine (both mentally and code-wise). We will also
> have an extra state/special value that can be repurposed (for swap entries
> that never gets re-duplicated).
>
> Another motivation (albeit a bit premature at the moment) is the new swap
> abstraction I am currently working on, that would allow for swap/zswap
> decoupling, swapoff optimization, etc. The fewer states and swap API
> functions there are, the simpler the conversion will be.
>
> I am sending this series first as an RFC, just in case I missed something
> or misunderstood this state, or if someone has a swap optimization in mind
> for shmem that would require this special state.

I have the same patch sitting in a tree somewhere from when I tried
working on swap abstraction, except then swap_shmem_alloc() did not
take a 'nr' argument so I did not need swap_duplicate_nr(). I was
going to send it out with other swap code cleanups I had, but I ended
up deciding to do nothing.

So for what it's worth I think this is correct:
Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>

>
> Swap experts, let me know if I'm mistaken :) Otherwise if there is no
> objection I will resend this patch series again for merging.
>
> Nhat Pham (2):
>   swapfile: add a batched variant for swap_duplicate()
>   swap: shmem: remove SWAP_MAP_SHMEM
>
>  include/linux/swap.h | 16 ++++++++--------
>  mm/shmem.c           |  2 +-
>  mm/swapfile.c        | 28 +++++++++-------------------
>  3 files changed, 18 insertions(+), 28 deletions(-)
>
>
> base-commit: acfabf7e197f7a5bedf4749dac1f39551417b049
> --
> 2.43.5