linux-kernel - Re: [PATCH v2 1/2] md: Fix overflow in is_mddev

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Message-ID: <CAPhsuW61yzkH5Tp0ku37DBjnQzT81yJUx0F4bag4xdq1rX5gsA@mail.gmail.com>
Date: Mon, 18 Dec 2023 08:04:50 -0800
From: Song Liu <song@...nel.org>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: linan666@...weicloud.com, axboe@...nel.dk, linux-raid@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-block@...r.kernel.org, 
	yi.zhang@...wei.com, houtao1@...wei.com, yangerkun@...wei.com, 
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2 1/2] md: Fix overflow in is_mddev_idle

On Sun, Dec 17, 2023 at 5:39 PM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
[...]
> >
> > We only use  this for idle or not check, the behavior is OK (I think).
> > However, this logic is error prone.
> >
> > On 64-bit systems, there is a 4-byte hole behind sync_io. I think we can
> > just use it for atomic64_t so that we don't have to worry about overflow.
>
> I'm not sure about this, because other than this ubsan warning, this
> overflow doesn't have any impact on functionality to me.

Fixing warnings for zero or low cost is always a good idea. It helps boost
the signal when UBSAN (and other debug features) detects real issues.

> If we care about this 'hole', there are lots of holes in gendisk, and
> can be avoiled, for example, moving 'sync_io' near  to 'node_id'.

The point was not "let's fill the hole", but "we can use atomic64_t
without extra memory cost". In general, I don't think we care too
much about holes in "struct gendisk".

Does this make sense?

Thanks,
Song