Re: [PATCH 4/4] xfs: report the correct read/write dio alignment for reflinked inodes

[John Garry <john.g.garry@oracle.com>]

On 07/01/2025 07:03, Darrick J. Wong wrote:
> On Tue, Jan 07, 2025 at 07:10:12AM +0100, Christoph Hellwig wrote:
>> On Mon, Jan 06, 2025 at 06:37:06PM +0000, John Garry wrote:
>>>> +	/*
>>>> +	 * On COW inodes we are forced to always rewrite an entire file system
>>>> +	 * block or RT extent.
>>>> +	 *
>>>> +	 * Because applications assume they can do sector sized direct writes
>>>> +	 * on XFS we fall back to buffered I/O for sub-block direct I/O in that
>>>> +	 * case.  Because that needs to copy the entire block into the buffer
>>>> +	 * cache it is highly inefficient and can easily lead to page cache
>>>> +	 * invalidation races.
>>>> +	 *
>>>> +	 * Tell applications to avoid this case by reporting the natively
>>>> +	 * supported direct I/O read alignment.
>>>
>>> Maybe I mis-read the complete comment, but did you really mean "natively
>>> supported direct I/O write alignment"? You have been talking about writes
>>> only, but then finally mention read alignment.
>>
>> No, this is indeed intended to talk about the different (smaller) read
>> alignment we are now reporting.  But I guess the wording is confusing
>> enough that I should improve it?

I know what you are saying, but I found the last paragraph odd. All 
along we talk about writes, but then conclude by mentioning reads (and 
not writes at all).

> 
> How about:
> 
> /*
>   * For COW inodes, we can only perform out of place writes of entire
>   * file allocation units (clusters).  For a sub-cluster directio write,
>   * we must fall back to buffered I/O to perform the RMW.  At best this
>   * is highly inefficient; at worst it leads to page cache invalidation
>   * races.  Tell applications to avoid this by reporting separately the
>   * read and (larger) write alignments.

yeah, that ending is better, but maybe Christoph wants to keep the more 
wordy original previous paragraphs.

>   */
> 
> --D
>