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955b4caa48
recover4 allocates 16 pages of memory via mmap, makes a 4 page hole in it with munmap, allocates another 16 pages of memory via normal allocation and then tries to copy from one to the other. For some reason on arm64 (but no other platform I have tested) the second allocation sometimes causes the runtime to ask the kernel for 4 additional pages of memory -- which the kernel satisfies by remapping the pages that were just unmapped! Moving the second allocation before the munmap fixes this behaviour, I can run recover4 tens of thousands of times without failure with this fix vs a failure rate of ~0.5% before. Fixes #12549 Change-Id: I490b895b606897e4f7f25b1b51f5d485a366fffb Reviewed-on: https://go-review.googlesource.com/14632 Reviewed-by: Dave Cheney <dave@cheney.net>
74 lines
2.2 KiB
Go
74 lines
2.2 KiB
Go
// +build linux darwin
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// run
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// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Test that if a slice access causes a fault, a deferred func
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// sees the most recent value of the variables it accesses.
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// This is true today; the role of the test is to ensure it stays true.
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//
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// In the test, memcopy is the function that will fault, during dst[i] = src[i].
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// The deferred func recovers from the error and returns, making memcopy
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// return the current value of n. If n is not being flushed to memory
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// after each modification, the result will be a stale value of n.
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//
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// The test is set up by mmapping a 64 kB block of memory and then
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// unmapping a 16 kB hole in the middle of it. Running memcopy
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// on the resulting slice will fault when it reaches the hole.
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package main
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import (
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"log"
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"runtime/debug"
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"syscall"
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"unsafe"
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)
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func memcopy(dst, src []byte) (n int, err error) {
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defer func() {
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err = recover().(error)
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}()
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for i := 0; i < len(dst) && i < len(src); i++ {
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dst[i] = src[i]
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n++
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}
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return
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}
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func main() {
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// Turn the eventual fault into a panic, not a program crash,
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// so that memcopy can recover.
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debug.SetPanicOnFault(true)
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size := syscall.Getpagesize()
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// Map 16 pages of data with a 4-page hole in the middle.
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data, err := syscall.Mmap(-1, 0, 16*size, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANON|syscall.MAP_PRIVATE)
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if err != nil {
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log.Fatalf("mmap: %v", err)
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}
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other := make([]byte, 16*size)
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// Note: Cannot call syscall.Munmap, because Munmap checks
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// that you are unmapping a whole region returned by Mmap.
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// We are trying to unmap just a hole in the middle.
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if _, _, err := syscall.Syscall(syscall.SYS_MUNMAP, uintptr(unsafe.Pointer(&data[8*size])), uintptr(4*size), 0); err != 0 {
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log.Fatalf("munmap: %v", err)
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}
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// Check that memcopy returns the actual amount copied
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// before the fault (8*size - 5, the offset we skip in the argument).
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n, err := memcopy(data[5:], other)
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if err == nil {
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log.Fatal("no error from memcopy across memory hole")
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}
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if n != 8*size-5 {
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log.Fatal("memcopy returned %d, want %d", n, 8*size-5)
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}
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}
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