mm: Allow architectures to request 'old' entries when prefaulting

Commit 5c0a85fad949 ("mm: make faultaround produce old ptes") changed
the "faultaround" behaviour to initialise prefaulted PTEs as 'old',
since this avoids vmscan wrongly assuming that they are hot, despite
having never been explicitly accessed by userspace. The change has been
shown to benefit numerous arm64 micro-architectures (with hardware
access flag) running Android, where both application launch latency and
direct reclaim time are significantly reduced (by 10%+ and ~80%
respectively).

Unfortunately, commit 315d09bf30c2 ("Revert "mm: make faultaround
produce old ptes"") reverted the change due to it being identified as
the cause of a ~6% regression in unixbench on x86. Experiments on a
variety of recent arm64 micro-architectures indicate that unixbench is
not affected by the original commit, which appears to yield a 0-1%
performance improvement.

Since one size does not fit all for the initial state of prefaulted
PTEs, introduce arch_wants_old_prefaulted_pte(), which allows an
architecture to opt-in to 'old' prefaulted PTEs at runtime based on
whatever criteria it may have.

Cc: Jan Kara <jack@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 19 insertions, 1 deletions

diff --git a/mm/memory.c b/mm/memory.c
index 3e2fc2950ad7..f0e7c589ca9d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -134,6 +134,18 @@ static inline bool arch_faults_on_old_pte(void)
 }
 #endif
 
+#ifndef arch_wants_old_prefaulted_pte
+static inline bool arch_wants_old_prefaulted_pte(void)
+{
+	/*
+	 * Transitioning a PTE from 'old' to 'young' can be expensive on
+	 * some architectures, even if it's performed in hardware. By
+	 * default, "false" means prefaulted entries will be 'young'.
+	 */
+	return false;
+}
+#endif
+
 static int __init disable_randmaps(char *s)
 {
 	randomize_va_space = 0;
@@ -3725,11 +3737,17 @@ void do_set_pte(struct vm_fault *vmf, struct page *page)
 {
 	struct vm_area_struct *vma = vmf->vma;
 	bool write = vmf->flags & FAULT_FLAG_WRITE;
+	bool prefault = vmf->flags & FAULT_FLAG_PREFAULT;
 	pte_t entry;
 
 	flush_icache_page(vma, page);
 	entry = mk_pte(page, vma->vm_page_prot);
-	entry = pte_sw_mkyoung(entry);
+
+	if (prefault && arch_wants_old_prefaulted_pte())
+		entry = pte_mkold(entry);
+	else
+		entry = pte_sw_mkyoung(entry);
+
 	if (write)
 		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
 	/* copy-on-write page */