linux-kernel - Re: [PATCH v3 RESEND 3/6] bitmap: Make bitmap

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Date: Mon, 12 Feb 2024 11:13:13 -0800
From: Yury Norov <yury.norov@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Herve Codina <herve.codina@...tlin.com>,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, Andrew Lunn <andrew@...n.ch>,
	Mark Brown <broonie@...nel.org>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v3 RESEND 3/6] bitmap: Make bitmap_onto() available to
 users

On Mon, Feb 12, 2024 at 04:36:36PM +0200, Andy Shevchenko wrote:
> On Mon, Feb 12, 2024 at 03:20:22PM +0100, Herve Codina wrote:
> > On Mon, 12 Feb 2024 16:01:38 +0200
> > Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> 
> ...
> 
> > Agree, the bitmap_onto() code is simpler to understand than its help.
> > 
> > I introduced bitmap_off() to be the "reverse" bitmap_onto() operations
> > and I preferred to avoid duplicating function that do the same things.
> > 
> > On my side, I initially didn't use the bitmap_*() functions and did the the
> > bits manipulation by hand.
> > During the review, it was suggested to use the bitmap_*() family and I followed
> > this suggestion.
> 
> I also would go this way, the problems I see with the current implementation are:

Sure, opencoding and duplicating the functionality is always a bad
idea.

> - being related to NUMA (and as Rasmus once pointed out better to be there);

It's 'related to NUMA' for the only reason - it's used by NUMA only.
Nothing NUMA-specific in the function itself.

Now that we've got a non-NUMA user, the bitmap_onto() is not related
to NUMA anymore.

> - unclear naming, esp. proposed bitmap_off();

That's I agree. Scatter/gather from your last approach sound better.
Do you plan to send a v2?

> - the quite hard to understand help text

Yes, we need a picture that would illustrate what actually happens

> - atomicity when it's not needed (AFAICT).

Agree. A series of atomic ops is not atomic. For example

        if (test_bit(n, map))
                set_bit(m, map);

is not atomic as a whole. And this is what we do in bitmap_onto/off()
in a loop. This must be fixed by using underscoded version.

> > I did tests to be sure that bitmap_onto() and bitmap_off() did
> > exactly the same things as my previous code did.
> 
> Yuri, what do you think about all this?

I think your scatter/gather is better then this onto/off by naming and
implementation. If you'll send a v2, and it would work for Herve, I'd
prefer scatter/gather. But we can live with onto/off as well.

Thanks,
Yury