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

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Date:   Fri, 1 Jul 2022 09:58:13 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     akpm@...ux-foundation.org, willy@...radead.org,
        aneesh.kumar@...ux.ibm.com, arnd@...db.de, 21cnbao@...il.com,
        corbet@....net, dave.hansen@...ux.intel.com, david@...hat.com,
        ebiederm@...ssion.com, hagen@...u.net, jack@...e.cz,
        keescook@...omium.org, kirill@...temov.name, kucharsk@...il.com,
        linkinjeon@...nel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        longpeng2@...wei.com, luto@...nel.org, markhemm@...glemail.com,
        pcc@...gle.com, rppt@...nel.org, sieberf@...zon.com,
        sjpark@...zon.de, surenb@...gle.com, tst@...oebel-theuer.de,
        yzaikin@...gle.com
Subject: Re: [PATCH v2 7/9] mm/mshare: Add unlink and munmap support

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@...cle.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