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

Khalid Aziz
2022-07-01T15:59:25Z

On 6/30/22 15:50, Darrick J. Wong wrote:
> 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.

Documentation (vfs.rst) says - "this method is called by destroy_inode() to release resources allocated for struct
inode. It is only required if ->alloc_inode was defined and simply undoes anything done by ->alloc_inode.". I am not
defining alloc_inode, so I assumed I do not need to define destroy_inode and the standard destroy_inode will do the
right thing since standard alloc_inode is being used.

Are you suggesting per-region mshare_data should be freed in destroy_inode instead of in close?

>
>> + } 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?

It looks like that is a bug currently. I need to account for that. Thanks!

--
Khalid