linux-kernel - RE: [PATCH v3 3/5] treewide: use get_random

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Message-ID: <e0c127f9e80146c19fab9f987bb2f588@AcuMS.aculab.com>
Date:   Fri, 7 Oct 2022 09:14:54 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        "Jason A. Donenfeld" <Jason@...c4.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>,
        Andreas Noever <andreas.noever@...il.com>,
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Subject: RE: [PATCH v3 3/5] treewide: use get_random_u32() when possible

From: Christophe Leroy
> Sent: 06 October 2022 18:43
...
> But taking into account that sp must remain 16 bytes aligned, would it
> be better to do something like ?
> 
> 	sp -= prandom_u32_max(PAGE_SIZE >> 4) << 4;

That makes me think...
If prandom_u32_max() is passed a (constant) power of 2 it doesn't
need to do the multiply, it can just do a shift right.

Doesn't it also always get a 32bit random value?
So actually get_random_u32() & PAGE_MASK & ~0xf is faster!

When PAGE_SIZE is 4k, PAGE_SIZE >> 4 is 256 so it could use:
	get_ramdom_u8() << 4

You also seem to have removed prandom_u32() in favour of
get_random_u32() but have added more prandom_xxxx() functions.

	David

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