Re: [PATCH v2 7/9] mm/mshare: Add unlink and munmap support

Darrick J. Wong
2022-06-30T21:50:26Z

On Wed, Jun 29, 2022 at 04:53:58PM -0600, Khalid Aziz wrote:
> Number of mappings of an mshare region should be tracked so it can
> be removed when there are no more references to it and associated
> file has been deleted. This add code to support the unlink operation
> for associated file, remove the mshare region on file deletion if
> refcount goes to zero, add munmap operation to maintain refcount
> to mshare region and remove it on last munmap if file has been
> deleted.
>
> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
> ---
> mm/mshare.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 63 insertions(+), 1 deletion(-)
>
> diff --git a/mm/mshare.c b/mm/mshare.c
> index 088a6cab1e93..90ce0564a138 100644
> --- a/mm/mshare.c
> +++ b/mm/mshare.c
> @@ -29,6 +29,7 @@ static struct super_block *msharefs_sb;
> struct mshare_data {
> struct mm_struct *mm;
> refcount_t refcnt;
> + int deleted;
> struct mshare_info *minfo;
> };
>
> @@ -48,6 +49,7 @@ msharefs_read(struct kiocb *iocb, struct iov_iter *iov)
> size_t ret;
> struct mshare_info m_info;
>
> + mmap_read_lock(info->mm);
> if (info->minfo != NULL) {
> m_info.start = info->minfo->start;
> m_info.size = info->minfo->size;
> @@ -55,18 +57,42 @@ msharefs_read(struct kiocb *iocb, struct iov_iter *iov)
> m_info.start = 0;
> m_info.size = 0;
> }
> + mmap_read_unlock(info->mm);
> ret = copy_to_iter(&m_info, sizeof(m_info), iov);
> if (!ret)
> return -EFAULT;
> return ret;
> }
>
> +static void
> +msharefs_close(struct vm_area_struct *vma)
> +{
> + struct mshare_data *info = vma->vm_private_data;
> +
> + if (refcount_dec_and_test(&info->refcnt)) {
> + mmap_read_lock(info->mm);
> + if (info->deleted) {
> + mmap_read_unlock(info->mm);
> + mmput(info->mm);
> + kfree(info->minfo);
> + kfree(info);

Aren't filesystems supposed to take care of disposing of the file data
in destroy_inode? IIRC struct inode doesn't go away until all fds are
closed, mappings are torn down, and there are no more references from
dentries. I could be misremembering since it's been a few months since
I went looking at the (VFS) inode lifecycle.

> + } else {
> + mmap_read_unlock(info->mm);
> + }
> + }
> +}
> +
> +static const struct vm_operations_struct msharefs_vm_ops = {
> + .close = msharefs_close,
> +};
> +
> static int
> msharefs_mmap(struct file *file, struct vm_area_struct *vma)
> {
> struct mshare_data *info = file->private_data;
> struct mm_struct *mm = info->mm;
>
> + mmap_write_lock(mm);
> /*
> * If this mshare region has been set up once already, bail out
> */
> @@ -80,10 +106,14 @@ msharefs_mmap(struct file *file, struct vm_area_struct *vma)
> mm->task_size = vma->vm_end - vma->vm_start;
> if (!mm->task_size)
> mm->task_size--;
> + mmap_write_unlock(mm);
> info->minfo->start = mm->mmap_base;
> info->minfo->size = mm->task_size;
> + info->deleted = 0;
> + refcount_inc(&info->refcnt);
> vma->vm_flags |= VM_SHARED_PT;
> vma->vm_private_data = info;
> + vma->vm_ops = &msharefs_vm_ops;
> return 0;
> }
>
> @@ -240,6 +270,38 @@ msharefs_mkdir(struct user_namespace *mnt_userns, struct inode *dir,
> return ret;
> }
>
> +static int
> +msharefs_unlink(struct inode *dir, struct dentry *dentry)
> +{
> + struct inode *inode = d_inode(dentry);
> + struct mshare_data *info = inode->i_private;
> +
> + /*
> + * Unmap the mshare region if it is still mapped in
> + */
> + vm_munmap(info->minfo->start, info->minfo->size);
> +
> + /*
> + * Mark msharefs file for deletion so it can not be opened
> + * and used for mshare mappings any more
> + */
> + simple_unlink(dir, dentry);
> + mmap_write_lock(info->mm);
> + info->deleted = 1;
> + mmap_write_unlock(info->mm);

What if the file is hardlinked?

--D

> +
> + /*
> + * Is this the last reference? If so, delete mshare region and
> + * remove the file
> + */
> + if (!refcount_dec_and_test(&info->refcnt)) {
> + mmput(info->mm);
> + kfree(info->minfo);
> + kfree(info);
> + }
> + return 0;
> +}
> +
> static const struct inode_operations msharefs_file_inode_ops = {
> .setattr = simple_setattr,
> .getattr = simple_getattr,
> @@ -248,7 +310,7 @@ static const struct inode_operations msharefs_dir_inode_ops = {
> .create = msharefs_create,
> .lookup = simple_lookup,
> .link = simple_link,
> - .unlink = simple_unlink,
> + .unlink = msharefs_unlink,
> .mkdir = msharefs_mkdir,
> .rmdir = simple_rmdir,
> .mknod = msharefs_mknod,
> --
> 2.32.0
>