linux-kernel - [PATCH 4/4] mm: Optimise vmf_anon_prepare() for VMAs without an anon

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Date: Fri, 26 Apr 2024 15:45:03 +0100
From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Suren Baghdasaryan <surenb@...gle.com>,
	Peter Xu <peterx@...hat.com>,
	Jann Horn <jannh@...gle.com>
Subject: [PATCH 4/4] 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@...radead.org>
Reviewed-by: Jann Horn <jannh@...gle.com>
Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
---
 mm/memory.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7dc112d3a7e4..b5453b86ec4b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3232,16 +3232,21 @@ 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;
 }
 
 /*
-- 
2.43.0