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Avoid fork on mount for overlay2 in common case

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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)
This commit is contained in:
Derek McGowan 2016-08-22 11:43:10 -07:00
parent 9a3e7d9e34
commit c13a985fa1
2 changed files with 30 additions and 7 deletions
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4
daemon/graphdriver/overlay2/mount.go
View file

@ -31,12 +31,12 @@ type mountOptions struct {
Flag uint32
}
func mountFrom(dir, device, target, mType, label string) error {
func mountFrom(dir, device, target, mType string, flags uintptr, label string) error {
options := &mountOptions{
Device: device,
Target: target,
Type: mType,
Flag: 0,
Flag: uint32(flags),
Label: label,
}

33
daemon/graphdriver/overlay2/overlay.go
View file

@ -409,13 +409,36 @@ func (d *Driver) Get(id string, mountLabel string) (s string, err error) {
}()
workDir := path.Join(dir, "work")
opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", string(lowers), path.Join(id, "diff"), path.Join(id, "work"))
mountLabel = label.FormatMountLabel(opts, mountLabel)
if len(mountLabel) > syscall.Getpagesize() {
return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountLabel))
splitLowers := strings.Split(string(lowers), ":")
absLowers := make([]string, len(splitLowers))
for i, s := range splitLowers {
absLowers[i] = path.Join(d.home, s)
}
opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", strings.Join(absLowers, ":"), path.Join(dir, "diff"), path.Join(dir, "work"))
mountData := label.FormatMountLabel(opts, mountLabel)
mount := syscall.Mount
mountTarget := mergedDir
pageSize := syscall.Getpagesize()
// Use relative paths and mountFrom when the mount data has exceeded
// the page size. The mount syscall fails if the mount data cannot
// fit within a page and relative links make the mount data much
// smaller at the expense of requiring a fork exec to chroot.
if len(mountData) > pageSize {
opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", string(lowers), path.Join(id, "diff"), path.Join(id, "work"))
mountData = label.FormatMountLabel(opts, mountLabel)
if len(mountData) > pageSize {
return "", fmt.Errorf("cannot mount layer, mount label too large %d", len(mountData))
}
mount = func(source string, target string, mType string, flags uintptr, label string) error {
return mountFrom(d.home, source, target, mType, flags, label)
}
mountTarget = path.Join(id, "merged")
}
if err := mountFrom(d.home, "overlay", path.Join(id, "merged"), "overlay", mountLabel); err != nil {
if err := mount("overlay", mountTarget, "overlay", 0, mountData); err != nil {
return "", fmt.Errorf("error creating overlay mount to %s: %v", mergedDir, err)
}