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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe