linux-kernel - Re: [PATCH v2 1/1] swap: shmem: remove SWAP_MAP

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Message-ID: <CAKEwX=OTq2HaEKGgM4n8M60xh217r=vKs4U-GGc83moS5pcZJA@mail.gmail.com>
Date: Tue, 1 Oct 2024 19:04:14 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosryahmed@...gle.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, 
	v-songbaohua@...o.com, linux-mm@...ck.org, kernel-team@...a.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] swap: shmem: remove SWAP_MAP_SHMEM

On Tue, Oct 1, 2024 at 6:58 PM Nhat Pham <nphamcs@...il.com> wrote:
>
> On Tue, Oct 1, 2024 at 6:33 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>
> I was debating between WARN-ing here, and returning -ENOMEM and
> WARN-ing at shmem's callsite.
>
> My thinking is that if we return -ENOMEM here, it will work in the
> current setup, for both shmem and other callsites. However, in the
> future, if we add another user of swap_duplicate_nr(), this time
> without guaranteeing that we won't need continuation, I think it won't
> work unless we have the fallback logic in place as well:
>
> while (!err && __swap_duplicate(entry, 1, nr) == -ENOMEM)
> err = add_swap_count_continuation(entry, GFP_ATOMIC);

Sorry, I accidentally sent out the email without completing my explanation :)

Anyway, the point being, with the current implementation, any new user
would immediately hit a WARN and the implementer will know to check.

Whereas if we return -ENOMEM in __swap_duplicate(), then I think we
would just hang, no? We only try to add swap count continuation to the
first entry only, which is not sufficient to fix the problem.

I can probably whip up the fallback logic here, but it would be dead,
untestable code (as it has no users, and I cannot even conceive one to
test it). And the swap abstraction might render all of this moot
anyway.