Re: [RFC PATCH v1 11/18] xfs: add async buffered write support

[Stefan Roesch <shr@fb.com>]

On 4/28/22 2:54 PM, Dave Chinner wrote:
> On Thu, Apr 28, 2022 at 12:58:59PM -0700, Stefan Roesch wrote:
>>
>>
>> On 4/26/22 3:56 PM, Dave Chinner wrote:
>>> On Tue, Apr 26, 2022 at 10:43:28AM -0700, Stefan Roesch wrote:
>>>> This adds the async buffered write support to XFS. For async buffered
>>>> write requests, the request will return -EAGAIN if the ilock cannot be
>>>> obtained immediately.
>>>>
>>>> Signed-off-by: Stefan Roesch <shr@fb.com>
>>>> ---
>>>>  fs/xfs/xfs_file.c | 10 ++++++----
>>>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
>>>> index 6f9da1059e8b..49d54b939502 100644
>>>> --- a/fs/xfs/xfs_file.c
>>>> +++ b/fs/xfs/xfs_file.c
>>>> @@ -739,12 +739,14 @@ xfs_file_buffered_write(
>>>>  	bool			cleared_space = false;
>>>>  	int			iolock;
>>>>  
>>>> -	if (iocb->ki_flags & IOCB_NOWAIT)
>>>> -		return -EOPNOTSUPP;
>>>> -
>>>>  write_retry:
>>>>  	iolock = XFS_IOLOCK_EXCL;
>>>> -	xfs_ilock(ip, iolock);
>>>> +	if (iocb->ki_flags & IOCB_NOWAIT) {
>>>> +		if (!xfs_ilock_nowait(ip, iolock))
>>>> +			return -EAGAIN;
>>>> +	} else {
>>>> +		xfs_ilock(ip, iolock);
>>>> +	}
>>>
>>> xfs_ilock_iocb().
>>>
>>
>> The helper xfs_ilock_iocb cannot be used as it hardcoded to use iocb->ki_filp to
>> get a pointer to the xfs_inode.
> 
> And the problem with that is?
> 
> I mean, look at what xfs_file_buffered_write() does to get the
> xfs_inode 10 lines about that change:
> 
> xfs_file_buffered_write(
>         struct kiocb            *iocb,
>         struct iov_iter         *from)
> {
>         struct file             *file = iocb->ki_filp;
>         struct address_space    *mapping = file->f_mapping;
>         struct inode            *inode = mapping->host;
>         struct xfs_inode        *ip = XFS_I(inode);
> 
> In what cases does file_inode(iocb->ki_filp) point to a different
> inode than iocb->ki_filp->f_mapping->host? The dio write path assumes
> that file_inode(iocb->ki_filp) is correct, as do both the buffered
> and dio read paths.
> 
> What makes the buffered write path special in that
> file_inode(iocb->ki_filp) is not correctly set whilst
> iocb->ki_filp->f_mapping->host is?
> 

In the function xfs_file_buffered_write() the code calls the function 
xfs_ilock(). The xfs_inode pointer that is passed in is iocb->ki_filp->f_mapping->host.
The one used in xfs_ilock_iocb is ki_filp->f_inode.

After getting the lock, the code in xfs_file_buffered_write calls the
function xfs_buffered_write_iomap_begin(). In this function the code
calls xfs_ilock() for ki_filp->f_inode in exclusive mode.

If I replace the first xfs_ilock() call with xfs_ilock_iocb(), then it looks
like I get a deadlock.


Am I missing something?

I can:
- replace the pointer to iocb with pointer to xfs_inode in the function xfs_ilock_iocb()
  and also pass in the flags value as a parameter.
or
- create function xfs_ilock_inode(), which xfs_ilock_iocb() calls. The existing
  calls will not need to change, only the xfs_ilock in xfs_file_buffered_write()
  will use xfs_ilock_inode().


> Regardless, if this is a problem, then just pass the XFS inode to
> xfs_ilock_iocb() and this is a moot point.
> 
>> However here we need to use iocb->ki_filp->f_mapping->host.
>> I'll split off new helper for this in the next version of the patch.
> 
> We don't need a new helper here, either.
> 
> Cheers,
> 
> Dave.