On Thu, Mar 29, 2018 at 10:38 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> If we find a upper metacopy inode, make sure we also found associated data
> dentry in lower. Otherwise copy up operation later will fail.
>
> There are two cases where this can happen. First case is that somehow
> data file was removed from lower layer. Other case is that REDIRECT
> xattr was removed due to copy up of file on another cpu (when inode is
> shared between two dentries) and hence ovl_lookup() could not find the
> correct dentry.
>
Remind me again why we remove REDIRECT xattr?
Is it a must for functionality or just for being boy scouts?
I would prefer to avoid having to deal with races of this sort.
You can cleanup REDIRECT for non-dir that is not metacopy
on lookup when finding a I_NEW inode.
> First case is an error while second case is not error. If file has been
> copied up, then it does not matter if data dentry was found or not.
>
> Redirect removal is protected using ovl_inode->lock and ovl_lookup() does
> not have access to that lock. So to differentiate between these two
> cases, take appropriate inode lock in ovl_get_inode() and make sure a
> data dentry has been found for metacopy inode. Otherwise lookup failed
> and its an error.
>
> For example, say two files are hardlinked, foo.txt and bar.txt. Say foo.txt
> is renamed to foo-renamed.txt gets copied up metadata only. This will also
> put a redirect "/foo.txt" on hardlnk inode. Now assume foo-renamed.txt
> is opened for write and is undergoing data copy up on one cpu and bar.txt
> is under going ovl_lookup() on other cpu. Data copy up path will remove
> REDIRECT and METACOPY xattr. It is possible that METACOPY xattr is
> visible to ovl_lookup() but by the REDIRECT xattr was gone by the time.
> That means no data dentry will be found but at the same time now inode
> is not metacopy inode. So data dentry is not required. So this is not
> error case. But if inode was still metacopy but data dentry was not found
> this is error case. (May be due to underlying layer changed). Fix it by
> returning -ESTALE.
>
> If inode was found in cache, then take ovl_inode->lock before checking
> status of inode. If inode has been allocated, then it is returned with
> inode lock anyway and other threads will block on that lock, so no need
> to take ovl_inode->lock.
>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> ---
> fs/overlayfs/export.c | 3 ++-
> fs/overlayfs/inode.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++-
> fs/overlayfs/namei.c | 14 ++++----------
> fs/overlayfs/overlayfs.h | 3 ++-
> 4 files changed, 56 insertions(+), 13 deletions(-)
>
> diff --git a/fs/overlayfs/export.c b/fs/overlayfs/export.c
> index 1c233096e59c..e8575d4d2c77 100644
> --- a/fs/overlayfs/export.c
> +++ b/fs/overlayfs/export.c
> @@ -305,7 +305,8 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
> if (d_is_dir(upper ?: lower))
> return ERR_PTR(-EIO);
>
> - inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower, NULL);
> + inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower, NULL,
> + false);
> if (IS_ERR(inode)) {
> dput(upper);
> return ERR_CAST(inode);
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 6a0c85699024..7e30f4a7cdd9 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -685,9 +685,42 @@ static bool ovl_hash_bylower(struct super_block *sb, struct dentry *upper,
> return true;
> }
>
> +static bool ovl_verify_metacopy_data(struct super_block *sb,
> + struct inode *inode, bool metacopydata)
> +{
> + struct ovl_fs *ofs = sb->s_fs_info;
> + struct ovl_inode *oi = OVL_I(inode);
> + bool metacopy = false;
> +
> + /* A metacopy data dentry was found. So no need to do further checks */
> + if (metacopydata)
> + return true;
> +
> + /*
> + * Metacopy feature not enabled. No metadata data copy up should take
> + * place. So no further checks needed.
> + */
> + if (!ofs->config.metacopy)
> + return true;
> +
> + if (!S_ISREG(inode->i_mode))
> + return true;
> +
> + /*
> + * Metacopy feature is enabled and we have not found metacopy data
> + * dentry. Make sure this inode is not metacopy inode.
> + */
> + mutex_lock(&oi->lock);
> + metacopy = !ovl_test_flag(OVL_UPPERDATA, inode);
> + mutex_unlock(&oi->lock);
> +
FYI, the reason you need mutex_lock_interruptible is so if other CPU
is busy with large copy up, the process that does lookup() can
be interrupted by user.
This is not likley to be a problem here, because race condition suggests that
copy up of data is complete by now, but nevertheless.
Thanks,
Amir.
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