linux-kernel - Re: [PATCH 1/1] alpha: Fix pte_swp

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Message-ID: <20250216172637.GI1977892@ZenIV>
Date: Sun, 16 Feb 2025 17:26:37 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Magnus Lindholm <linmag7@...il.com>
Cc: richard.henderson@...aro.org, mattst88@...il.com,
	glaubitz@...sik.fu-berlin.de, ink@...een.parts, kees@...nel.org,
	arnd@...db.de, linux-kernel@...r.kernel.org,
	linux-alpha@...r.kernel.org
Subject: Re: [PATCH 1/1] alpha: Fix pte_swp_exclusive on alpha

On Sun, Feb 16, 2025 at 05:17:41PM +0000, Al Viro wrote:
> On Sun, Feb 16, 2025 at 06:04:53PM +0100, Magnus Lindholm wrote:
> > Function pte_swp_exclusive() checks if _PAGE_SWP_EXCLUSIVE bit is set in
> > PTE but returns lower 32-bits only. Shift bits right by 32 to return upper
> > 32-bits of PTE which contain the _PAGE_SWP_EXCLUSIVE bit. On alpha this is
> > bit 39 but on most other architectures this bit already resides somewhere
> > in the first 32-bits and hence a shift is not necessary on those archs.
> 
> Just make it return bool and be done with that - all users are either
> if (pte_swp_exclusive(...)) or if (!pte_swp_exclusive(...)) or assignments
> to bool variable.
> 
> No need to shift anything - compiler probably will figure out that
> 	if ((int)((x & (1UL<<39)>>32)))

Sorry,

	if ((int)(((x & (1UL<<39))>>32))

> is equivalent to
> 	if (x & (1UL<<39))
> but why bother with such convolutions in the first place?
> 
> Seriously, just make it
> 
> bool pte_swp_exclusive(pte_t pte)
> {
> 	return pte_val(pte) & _PAGE_SWP_EXCLUSIVE;
> }
> 
> and that's it - conversion from arithmetical types to bool will do the right thing.
>