linux-kernel - Re: [PATCH] mmzone: remove redundant exception handler on pfn

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Message-ID: <ZvRPfiS9KTi7NPV4@casper.infradead.org>
Date: Wed, 25 Sep 2024 18:59:26 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Leesoo Ahn <lsahn@...eel.net>
Cc: lsahn@...akecorp.com, Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmzone: remove redundant exception handler on pfn_valid()

On Thu, Sep 26, 2024 at 12:55:28AM +0900, Leesoo Ahn wrote:
> 1). It can ensure the upper PAGE_SHIFT'ed bits of PHYS_PFN(..) are clear
> by the C11 standard:

I don't think you understand the purpose of the code you're removing.

> +++ b/include/linux/mmzone.h
> @@ -2028,15 +2028,6 @@ static inline int pfn_valid(unsigned long pfn)
>  	struct mem_section *ms;
>  	int ret;
>  
> -	/*
> -	 * Ensure the upper PAGE_SHIFT bits are clear in the
> -	 * pfn. Else it might lead to false positives when
> -	 * some of the upper bits are set, but the lower bits
> -	 * match a valid pfn.
> -	 */
> -	if (PHYS_PFN(PFN_PHYS(pfn)) != pfn)
> -		return 0;

Let's assume that the pfn being asked about is 0x8000'0000'0000'1234.
The actually valid PFNs are only 0-0x1000'0000.  This function should
return false.  With your change, it will retrun true because it'll
only check the bottom 52 bits, and ignore that some of the top 12 bits
are set.