From: Matthew Wilcox (Oracle) <willy@infradead.org>
Date: Mon, 15 Apr 2024 15:13:41 +0000 (-0400)
Subject: mm: Optimise vmf_anon_prepare() for VMAs without an anon_vma
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=refs%2Fheads%2Fvma-lock;p=users%2Fwilly%2Fpagecache.git

mm: Optimise vmf_anon_prepare() for VMAs without an anon_vma

If the mmap_lock can be taken for read, we can call __anon_vma_prepare()
while holding it, saving ourselves a trip back through the fault handler.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jann Horn <jannh@google.com>
---

diff --git a/mm/memory.c b/mm/memory.c
index 64d2986dd2392..5624b881b662d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3224,16 +3224,21 @@ static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf)
 vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
+	vm_fault_t ret = 0;
 
 	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;
-	return 0;
+		ret = VM_FAULT_OOM;
+	if (vmf->flags & FAULT_FLAG_VMA_LOCK)
+		mmap_read_unlock(vma->vm_mm);
+	return ret;
 }
 
 /*