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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250401143520.1113572-1-mjguzik@gmail.com

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 296d294142c8d53f77221f1d4274b56c476387fc..697432f63c59ca50ebb407bfb03340c02433a0e0 100644 (file)
--- 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