linux-kernel - [PATCH 1/2] mm: vmalloc: Avoid a double lookup of freed VA in a tree

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Message-Id: <20221220182704.181657-1-urezki@gmail.com>
Date:   Tue, 20 Dec 2022 19:27:03 +0100
From:   "Uladzislau Rezki (Sony)" <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Baoquan He <bhe@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Uladzislau Rezki <urezki@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: [PATCH 1/2] mm: vmalloc: Avoid a double lookup of freed VA in a tree

When a VA is freed over a main path, for example by invoking
the vfree() function, a tree is accessed two times what is odd:

vfree():
  __vunmap()
    __find_vmap_area()
  vm_remove_mappings()
    remove_vm_area()
      __find_vmap_area()

__find_vmap_area() are called two times. Fix it by introducing
a find_unlink_vmap_area() helper that finds and un-links a VA
from a tree.

Performance test results on a single CPU:

- fix_size_alloc_test       loops: 1000000 avg: 476847   usec
- full_fit_alloc_test       loops: 1000000 avg: 806746   usec
- long_busy_list_alloc_test loops: 1000000 avg: 13552093 usec
- random_size_alloc_test    loops: 1000000 avg: 7441322  usec
- fix_align_alloc_test      loops: 1000000 avg: 1411132  usec
All test took worker0=87650866284 cycles

- fix_size_alloc_test       loops: 1000000 avg: 490713   usec
- full_fit_alloc_test       loops: 1000000 avg: 579162   usec
- long_busy_list_alloc_test loops: 1000000 avg: 10485448 usec
- random_size_alloc_test    loops: 1000000 avg: 5824449  usec
- fix_align_alloc_test      loops: 1000000 avg: 984735   usec
All test took worker0=67952362802 cycles

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
---
 mm/vmalloc.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 9e30f0b39203..0fc38c36e0df 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -1825,9 +1825,14 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 	unsigned long va_start = va->va_start;
 	unsigned long nr_lazy;
 
-	spin_lock(&vmap_area_lock);
-	unlink_va(va, &vmap_area_root);
-	spin_unlock(&vmap_area_lock);
+	/*
+	 * A free_vmap_block() is left. It is NOT a main free path.
+	 */
+	if (!list_empty(&va->list)) {
+		spin_lock(&vmap_area_lock);
+		unlink_va(va, &vmap_area_root);
+		spin_unlock(&vmap_area_lock);
+	}
 
 	nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
 				PAGE_SHIFT, &vmap_lazy_nr);
@@ -1871,6 +1876,19 @@ struct vmap_area *find_vmap_area(unsigned long addr)
 	return va;
 }
 
+static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
+{
+	struct vmap_area *va;
+
+	spin_lock(&vmap_area_lock);
+	va = __find_vmap_area(addr, &vmap_area_root);
+	if (va)
+		unlink_va(va, &vmap_area_root);
+	spin_unlock(&vmap_area_lock);
+
+	return va;
+}
+
 /*** Per cpu kva allocator ***/
 
 /*
@@ -2236,7 +2254,7 @@ void vm_unmap_ram(const void *mem, unsigned int count)
 		return;
 	}
 
-	va = find_vmap_area(addr);
+	va = find_unlink_vmap_area(addr);
 	BUG_ON(!va);
 	debug_check_no_locks_freed((void *)va->va_start,
 				    (va->va_end - va->va_start));
@@ -2607,21 +2625,16 @@ struct vm_struct *remove_vm_area(const void *addr)
 
 	might_sleep();
 
-	spin_lock(&vmap_area_lock);
-	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
-	if (va && va->vm) {
+	va = find_unlink_vmap_area((unsigned long) addr);
+	if (va) {
 		struct vm_struct *vm = va->vm;
 
-		va->vm = NULL;
-		spin_unlock(&vmap_area_lock);
-
 		kasan_free_module_shadow(vm);
 		free_unmap_vmap_area(va);
 
 		return vm;
 	}
 
-	spin_unlock(&vmap_area_lock);
 	return NULL;
 }
 
@@ -2690,6 +2703,7 @@ static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
 static void __vunmap(const void *addr, int deallocate_pages)
 {
 	struct vm_struct *area;
+	struct vmap_area *va;
 
 	if (!addr)
 		return;
@@ -2698,7 +2712,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
 			addr))
 		return;
 
-	area = find_vm_area(addr);
+	va = find_unlink_vmap_area((unsigned long)addr);
+	area = va->vm;
+
 	if (unlikely(!area)) {
 		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
 				addr);
-- 
2.30.2