Browse code
Avoid fork on mount for overlay2 in common case
In the common case where the user is using /var/lib/docker and
an image with less than 60 layers, forking is not needed. Calculate
whether absolute paths can be used and avoid forking to mount in
those cases.
Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Derek McGowan authored on 2016/08/23 03:43:10Showing 2 changed files
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@@ -31,12 +31,12 @@ type mountOptions struct {
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| 31 | 31 |
Flag uint32 |
| 32 | 32 |
} |
| 33 | 33 |
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-func mountFrom(dir, device, target, mType, label string) error {
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+func mountFrom(dir, device, target, mType string, flags uintptr, label string) error {
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options := &mountOptions{
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Device: device, |
| 37 | 37 |
Target: target, |
| 38 | 38 |
Type: mType, |
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- Flag: 0, |
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+ Flag: uint32(flags), |
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| 40 | 40 |
Label: label, |
| 41 | 41 |
} |
| 42 | 42 |
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| ... | ... |
@@ -409,13 +409,36 @@ func (d *Driver) Get(id string, mountLabel string) (s string, err error) {
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| 409 | 409 |
}() |
| 410 | 410 |
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| 411 | 411 |
workDir := path.Join(dir, "work") |
| 412 |
- opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", string(lowers), path.Join(id, "diff"), path.Join(id, "work"))
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- mountLabel = label.FormatMountLabel(opts, mountLabel) |
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- if len(mountLabel) > syscall.Getpagesize() {
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- return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountLabel))
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+ splitLowers := strings.Split(string(lowers), ":") |
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+ absLowers := make([]string, len(splitLowers)) |
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+ for i, s := range splitLowers {
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+ absLowers[i] = path.Join(d.home, s) |
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+ } |
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+ opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", strings.Join(absLowers, ":"), path.Join(dir, "diff"), path.Join(dir, "work"))
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+ mountData := label.FormatMountLabel(opts, mountLabel) |
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+ mount := syscall.Mount |
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+ mountTarget := mergedDir |
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+ |
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+ pageSize := syscall.Getpagesize() |
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+ |
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+ // Use relative paths and mountFrom when the mount data has exceeded |
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+ // the page size. The mount syscall fails if the mount data cannot |
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+ // fit within a page and relative links make the mount data much |
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+ // smaller at the expense of requiring a fork exec to chroot. |
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+ if len(mountData) > pageSize {
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+ opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", string(lowers), path.Join(id, "diff"), path.Join(id, "work"))
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+ mountData = label.FormatMountLabel(opts, mountLabel) |
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+ if len(mountData) > pageSize {
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+ return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountData))
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+ } |
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+ |
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+ mount = func(source string, target string, mType string, flags uintptr, label string) error {
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+ return mountFrom(d.home, source, target, mType, flags, label) |
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+ } |
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+ mountTarget = path.Join(id, "merged") |
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| 416 | 439 |
} |
| 417 | 440 |
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- if err := mountFrom(d.home, "overlay", path.Join(id, "merged"), "overlay", mountLabel); err != nil {
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+ if err := mount("overlay", mountTarget, "overlay", 0, mountData); err != nil {
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return "", fmt.Errorf("error creating overlay mount to %s: %v", mergedDir, err)
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| 420 | 443 |
} |
| 421 | 444 |
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