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

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Message-ID: <20240925155540.4087150-1-lsahn@ooseel.net>
Date: Thu, 26 Sep 2024 00:55:28 +0900
From: Leesoo Ahn <lsahn@...eel.net>
To: lsahn@...akecorp.com
Cc: Leesoo Ahn <lsahn@...eel.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] mmzone: remove redundant exception handler on pfn_valid()

1). It can ensure the upper PAGE_SHIFT'ed bits of PHYS_PFN(..) are clear
by the C11 standard:

    6.5.7 Bitwise shift operator

    ...

    The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has
    an unsigned type or if E1 has a signed type and a nonnegative value,
    the value of the result is the integral part of the quotient of E1 / 2^E2.

@pfn parameter of pfn_valid(..) is unsigned long and PHYS_PFN(..) makes
the upper bits clear(zero) always with it. Otherwise, the result of E1 / 2^E2
doesn't make sense in the document.

2). x86 instructions

The following x86 instructions are the part of the redundant exception handler:

    ...
    movabs  $0xfffffffffffff,%rax
    and     %rdi,%rax
    cmp     %rax,%rdi
    je      22
    xor     %eax,%eax
    jmp     __x86_return_thunk
22: push    %rbp
    ...

%rdi would be the @pfn parameter of pfn_valid(..) and -1 assigned to %rax.
'je' always takes place by %rdi AND and CMP ops with %rax.

Therefore, the exception handler is unnecessary.

Signed-off-by: Leesoo Ahn <lsahn@...eel.net>
---
 include/linux/mmzone.h | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 17506e4a2835..164ae80977d6 100644
--- a/include/linux/mmzone.h
+++ 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;
-
 	if (pfn_to_section_nr(pfn) >= NR_MEM_SECTIONS)
 		return 0;
 	ms = __pfn_to_section(pfn);
-- 
2.46.0