linux-kernel - [tip: x86/mm] x86/mm: Stop prefetching current->mm->mmap

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Message-ID: <174353304768.14745.8808578254812639633.tip-bot2@tip-bot2>
Date: Tue, 01 Apr 2025 18:44:07 -0000
From: "tip-bot2 for Mateusz Guzik" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Mateusz Guzik <mjguzik@...il.com>, Ingo Molnar <mingo@...nel.org>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Rik van Riel <riel@...riel.com>, "H. Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject:
 [tip: x86/mm] x86/mm: Stop prefetching current->mm->mmap_lock on page faults

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     0f0a9bf602449c0114117a72eab4329c9a22176d
Gitweb:        https://git.kernel.org/tip/0f0a9bf602449c0114117a72eab4329c9a22176d
Author:        Mateusz Guzik <mjguzik@...il.com>
AuthorDate:    Tue, 01 Apr 2025 16:35:20 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 01 Apr 2025 20:26:35 +02:00

x86/mm: Stop prefetching current->mm->mmap_lock on page faults

The prefetchw() dates back decades and the fundamental notion of doing
something like this on a lock is shady.

Moreover, for a few years now in the fast path faults are handled with RCU
+ per-vma locking, hopefully not even looking at the lock to begin with.

As such just remove it.

I did not see a point benchmarking this. Given that it is not expected
to be looked at by default justifies not doing the prefetch.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...riel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20250401143520.1113572-1-mjguzik@gmail.com
---
 arch/x86/mm/fault.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 296d294..697432f 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -13,7 +13,6 @@
 #include <linux/mmiotrace.h>		/* kmmio_handler, ...		*/
 #include <linux/perf_event.h>		/* perf_sw_event		*/
 #include <linux/hugetlb.h>		/* hstate_index_to_shift	*/
-#include <linux/prefetch.h>		/* prefetchw			*/
 #include <linux/context_tracking.h>	/* exception_enter(), ...	*/
 #include <linux/uaccess.h>		/* faulthandler_disabled()	*/
 #include <linux/efi.h>			/* efi_crash_gracefully_on_page_fault()*/
@@ -1496,8 +1495,6 @@ DEFINE_IDTENTRY_RAW_ERRORCODE(exc_page_fault)

 	address = cpu_feature_enabled(X86_FEATURE_FRED) ? fred_event_data(regs) : read_cr2();

-	prefetchw(&current->mm->mmap_lock);
-
 	/*
 	 * KVM uses #PF vector to deliver 'page not present' events to guests
 	 * (asynchronous page fault mechanism). The event happens when a