Re: [PATCH 5/5] iomap: support RWF_UNCACHED for buffered writes

[Jens Axboe <axboe@kernel.dk>]

On 12/12/19 5:54 PM, Jens Axboe wrote:
> On 12/12/19 3:34 PM, Dave Chinner wrote:
>> On Wed, Dec 11, 2019 at 08:29:43AM -0700, Jens Axboe wrote:
>>> This adds support for RWF_UNCACHED for file systems using iomap to
>>> perform buffered writes. We use the generic infrastructure for this,
>>> by tracking pages we created and calling write_drop_cached_pages()
>>> to issue writeback and prune those pages.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> ---
>>>  fs/iomap/apply.c       | 24 ++++++++++++++++++++++++
>>>  fs/iomap/buffered-io.c | 37 +++++++++++++++++++++++++++++--------
>>>  include/linux/iomap.h  |  5 +++++
>>>  3 files changed, 58 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/fs/iomap/apply.c b/fs/iomap/apply.c
>>> index 562536da8a13..966826ad4bb9 100644
>>> --- a/fs/iomap/apply.c
>>> +++ b/fs/iomap/apply.c
>>> @@ -90,5 +90,29 @@ iomap_apply(struct inode *inode, loff_t pos, loff_t length, unsigned flags,
>>>  				     flags, &iomap);
>>>  	}
>>>  
>>> +	if (written && (flags & IOMAP_UNCACHED)) {
>>> +		struct address_space *mapping = inode->i_mapping;
>>> +
>>> +		end = pos + written;
>>> +		ret = filemap_write_and_wait_range(mapping, pos, end);
>>> +		if (ret)
>>> +			goto out;
>>> +
>>> +		/*
>>> +		 * No pages were created for this range, we're done
>>> +		 */
>>> +		if (!(iomap.flags & IOMAP_F_PAGE_CREATE))
>>> +			goto out;
>>> +
>>> +		/*
>>> +		 * Try to invalidate cache pages for the range we just wrote.
>>> +		 * We don't care if invalidation fails as the write has still
>>> +		 * worked and leaving clean uptodate pages in the page cache
>>> +		 * isn't a corruption vector for uncached IO.
>>> +		 */
>>> +		invalidate_inode_pages2_range(mapping,
>>> +				pos >> PAGE_SHIFT, end >> PAGE_SHIFT);
>>> +	}
>>> +out:
>>>  	return written ? written : ret;
>>>  }
>>
>> Just a thought on further optimisation for this for XFS.
>> IOMAP_UNCACHED is being passed into the filesystem ->iomap_begin
>> methods by iomap_apply().  Hence the filesystems know that it is
>> an uncached IO that is being done, and we can tailor allocation
>> strategies to suit the fact that the data is going to be written
>> immediately.
>>
>> In this case, XFS needs to treat it the same way it treats direct
>> IO. That is, we do immediate unwritten extent allocation rather than
>> delayed allocation. This will reduce the allocation overhead and
>> will optimise for immediate IO locality rather than optimise for
>> delayed allocation.
>>
>> This should just be a relatively simple change to
>> xfs_file_iomap_begin() along the lines of:
>>
>> -	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) && !(flags & IOMAP_DIRECT) &&
>> -			!IS_DAX(inode) && !xfs_get_extsz_hint(ip)) {
>> +	if ((flags & (IOMAP_WRITE | IOMAP_ZERO)) &&
>> +	    !(flags & (IOMAP_DIRECT | IOMAP_UNCACHED)) &&
>> +	    !IS_DAX(inode) && !xfs_get_extsz_hint(ip)) {
>> 		/* Reserve delalloc blocks for regular writeback. */
>> 		return xfs_file_iomap_begin_delay(inode, offset, length, flags,
>> 				iomap);
>> 	}
>>
>> so that it avoids delayed allocation for uncached IO...
> 
> That's very handy! Thanks, I'll add that to the next version. Just out
> of curiosity, would you prefer this as a separate patch, or just bundle
> it with the iomap buffered RWF_UNCACHED patch? I'm assuming the latter,
> and I'll just mention it in the changelog.

OK, since it's in XFS, it'd be a separate patch. The code you quote seems
to be something out-of-tree?

-- 
Jens Axboe