[RFC] Trylock the mmap_lock in vmf_anon_prepare()

[Matthew Wilcox <willy@infradead.org>]

It seems a shame to fallback if the mmap_lock is readily available.
It doesn't blow up immediately in my testing ... thoughts?

From 70833af858ce41a80b5f0669648ee9f59b149a03 Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Mon, 15 Apr 2024 11:13:41 -0400
Subject: [PATCH] mm: Optimise vmf_anon_prepare() for VMAs without an anon_vma

If the mmap_lock can be taken for read, there's no reason not to take
it so that we can call __anon_vma_prepare().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/memory.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index b9c23393fa9b..7075fb7f74da 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3219,11 +3219,15 @@ vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
 	if (likely(vma->anon_vma))
 		return 0;
 	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-		vma_end_read(vma);
-		return VM_FAULT_RETRY;
+		if (!mmap_read_trylock(vma->vm_mm)) {
+			vma_end_read(vma);
+			return VM_FAULT_RETRY;
+		}
 	}
 	if (__anon_vma_prepare(vma))
 		return VM_FAULT_OOM;
+	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
+		mmap_read_unlock(vma->vm_mm);
 	return 0;
 }
 
-- 
2.43.0