linux-ext4 - Re: [PATCH 7/8] nowait aio: xfs

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Date:   Fri, 7 Apr 2017 06:34:28 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, jack@...e.com,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        sagi@...mberg.me, avi@...lladb.com, axboe@...nel.dk,
        linux-api@...r.kernel.org, willy@...radead.org,
        tom.leiming@...il.com, Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 7/8] nowait aio: xfs



On 04/06/2017 05:54 PM, Darrick J. Wong wrote:
> On Mon, Apr 03, 2017 at 11:52:11PM -0700, Christoph Hellwig wrote:
>>> +	if (unaligned_io) {
>>> +		/* If we are going to wait for other DIO to finish, bail */
>>> +		if ((iocb->ki_flags & IOCB_NOWAIT) &&
>>> +		     atomic_read(&inode->i_dio_count))
>>> +			return -EAGAIN;
>>>  		inode_dio_wait(inode);
>>
>> This checks i_dio_count twice in the nowait case, I think it should be:
>>
>> 	if (iocb->ki_flags & IOCB_NOWAIT) {
>> 		if (atomic_read(&inode->i_dio_count))
>> 			return -EAGAIN;
>> 	} else {
>> 		inode_dio_wait(inode);
>> 	}
>>
>>>  	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && xfs_is_reflink_inode(ip)) {
>>>  		if (flags & IOMAP_DIRECT) {
>>> +			/* A reflinked inode will result in CoW alloc */
>>> +			if (flags & IOMAP_NOWAIT) {
>>> +				error = -EAGAIN;
>>> +				goto out_unlock;
>>> +			}
>>
>> This is a bit pessimistic - just because the inode has any shared
>> extents we could still write into unshared ones.  For now I think this
>> pessimistic check is fine, but the comment should be corrected.
> 
> Consider what happens in both _reflink_{allocate,reserve}_cow.  If there
> is already an existing reservation in the CoW fork then we'll have to
> CoW and therefore can't satisfy the NOWAIT flag.  If there isn't already
> anything in the CoW fork, then we have to see if there are shared blocks
> by calling _reflink_trim_around_shared.  That performs a refcountbt
> lookup, which involves locking the AGF, so we also can't satisfy NOWAIT.
> 
> IOWs, I think this hunk has to move outside the IOMAP_DIRECT check to
> cover both write-to-reflinked-file cases.
> 

IOMAP_NOWAIT is set only with IOMAP_DIRECT since the nowait feature is
for direct-IO only. This is checked early on, when we are checking for
user-passed flags, and if not, -EINVAL is returned.


-- 
Goldwyn