On 06/22/2015 04:37 PM, Miklos Szeredi wrote:
On Mon, Jun 22, 2015 at 4:26 PM, Jan Olszak <j.olszak@xxxxxxxxxxx> wrote:
You can bind mount individual files from the lower layer to the
overlay. That fixes the "allow modification" part.
Well bind mounting every file that should have COW is unmaintainable - if
new files appear admin has to mount each one.
No. I was proposing bind mounts for non-COW ones.
mount -t overlay -olowerdir=/lower,upperdir=/upper,workdir=/work overlay /ovl
mount --bind /lower/this-file-is-non-cow /ovl/this-file-is-non-cow
...
Yes, you are right. This should work, but introduces an overhead of many
mounts for each Linux container.
The proposed alternative is: Use setfattr only once for each file and
then forget about it.
But what should happen on rename or unlink?
Both would operate on the original inode.
And that would make this hugely complex.
Are you sure? I haven't looked into rename and unlink routines but this
patch covers opening the file and is pretty simple (though it probably
breaks fchmod/fchown...).
I can try to prepare a real solution if you like the idea.
From 20e590c0e37f087955b25edfbc2ab4c9c034b5b1 Mon Sep 17 00:00:00 2001
From: Jan Olszak <j.olszak@xxxxxxxxxxx>
Date: Mon, 22 Jun 2015 17:07:31 +0200
Subject: [PATCH] Optional copy on write in overlayfs
---
fs/overlayfs/inode.c | 17 +++++++++++++++++
fs/overlayfs/overlayfs.h | 1 +
fs/overlayfs/super.c | 5 -----
3 files changed, 18 insertions(+), 5 deletions(-)
diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 04f1248..4f9f399 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -321,6 +321,20 @@ out:
return err;
}
+static bool ovl_dentry_need_copy_up(struct dentry *realdentry)
+{
+ int res;
+ char val;
+ struct inode *inode = realdentry->d_inode;
+
+ res = inode->i_op->getxattr(realdentry, OVL_XATTR_COW, &val, 1);
+ if (res == 1 && val == 'n'){
+ return false;
+ }
+
+ return true;
+}
+
static bool ovl_open_need_copy_up(int flags, enum ovl_path_type type,
struct dentry *realdentry)
{
@@ -330,6 +344,9 @@ static bool ovl_open_need_copy_up(int flags, enum
ovl_path_type type,
if (special_file(realdentry->d_inode->i_mode))
return false;
+ if (!ovl_dentry_need_copy_up(realdentry))
+ return false;
+
if (!(OPEN_FMODE(flags) & FMODE_WRITE) && !(flags & O_TRUNC))
return false;
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 17ac5af..b364888 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -26,6 +26,7 @@ enum ovl_path_type {
#define OVL_XATTR_PRE_NAME "trusted.overlay."
#define OVL_XATTR_PRE_LEN 16
#define OVL_XATTR_OPAQUE OVL_XATTR_PRE_NAME"opaque"
+#define OVL_XATTR_COW OVL_XATTR_PRE_NAME"cow"
static inline int ovl_do_rmdir(struct inode *dir, struct dentry *dentry)
{
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index bf8537c..916a4b5 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -944,11 +944,6 @@ static int ovl_fill_super(struct super_block *sb,
void *data, int silent)
pr_err("overlayfs: failed to clone lowerpath\n");
goto out_put_lower_mnt;
}
- /*
- * Make lower_mnt R/O. That way fchmod/fchown on lower file
- * will fail instead of modifying lower fs.
- */
- mnt->mnt_flags |= MNT_READONLY;
ufs->lower_mnt[ufs->numlower] = mnt;
ufs->numlower++;
--
1.9.1
Thanks,
Jan
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