linux-ext4 - Re: [PATCH v6 00/11] ext4: port direct I/O to iomap infrastructure

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Date:   Thu, 31 Oct 2019 17:54:16 +0100
From:   Jan Kara <jack@...e.cz>
To:     Matthew Bobrowski <mbobrowski@...browski.org>
Cc:     Jan Kara <jack@...e.cz>, "Theodore Y. Ts'o" <tytso@....edu>,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, hch@...radead.org,
        david@...morbit.com, darrick.wong@...cle.com
Subject: Re: [PATCH v6 00/11] ext4: port direct I/O to iomap infrastructure

On Thu 31-10-19 20:16:41, Matthew Bobrowski wrote:
> On Wed, Oct 30, 2019 at 12:39:18PM +0100, Jan Kara wrote:
> > On Wed 30-10-19 12:26:52, Jan Kara wrote:
> > > On Wed 30-10-19 13:00:24, Matthew Bobrowski wrote:
> > > > On Tue, Oct 29, 2019 at 07:34:01PM -0400, Theodore Y. Ts'o wrote:
> > > > > On Tue, Oct 29, 2019 at 07:31:59PM -0400, Theodore Y. Ts'o wrote:
> > > > > > Hi Matthew, it looks like there are a number of problems with this
> > > > > > patch series when using the ext3 backwards compatibility mode (e.g.,
> > > > > > no extents enabled).
> > > > > > 
> > > > > > So the following configurations are failing:
> > > > > > 
> > > > > > kvm-xfstests -c ext3   generic/091 generic/240 generic/263
> > > > 
> > > > This is one mode that I didn't get around to testing. Let me take a
> > > > look at the above and get back to you.
> > > 
> > > If I should guess, I'd start looking at what that -ENOTBLK fallback from
> > > direct IO ends up doing as we seem to be hitting that path...
> > 
> > Hum, actually no. This write from fsx output:
> > 
> > 24( 24 mod 256): WRITE    0x23000 thru 0x285ff  (0x5600 bytes)
> > 
> > should have allocated blocks to where the failed write was going (0x24000).
> > But still I'd expect some interaction between how buffered writes to holes
> > interact with following direct IO writes... One of the subtle differences
> > we have introduced with iomap conversion is that the old code in
> > __generic_file_write_iter() did fsync & invalidate written range after
> > buffered write fallback and we don't seem to do that now (probably should
> > be fixed regardless of relation to this bug).
> 
> After performing some debugging this afternoon, I quickly realised
> that the fix for this is rather trivial. Within the previous direct
> I/O implementation, we passed EXT4_GET_BLOCKS_CREATE to
> ext4_map_blocks() for any writes to inodes without extents. I seem to
> have missed that here and consequently block allocation for a write
> wasn't performing correctly in such cases.

No, this is not correct. For inodes without extents we used
ext4_dio_get_block() and we pass DIO_SKIP_HOLES to __blockdev_direct_IO().
Now DIO_SKIP_HOLES means that if starting block is within i_size, we pass
'create == 0' to get_blocks() function and thus ext4_dio_get_block() uses
'0' argument to ext4_map_blocks() similarly to what you do.

And indeed for inodes without extents we must fallback to buffered IO for
filling holes inside a file to avoid stale data exposure (racing DIO read
could read block contents before data is written to it if we used
EXT4_GET_BLOCKS_CREATE).

> Also, I agree, the fsync + page cache invalidation bits need to be
> implemented. I'm just thinking to branch out within
> ext4_buffered_write_iter() and implement those bits there i.e.
> 
> 	...
> 	ret = generic_perform_write();
> 
> 	if (ret > 0 && iocb->ki_flags & IOCB_DIRECT) {
> 	   	err = filemap_write_and_wait_range();
> 
> 		if (!err)
> 			invalidate_mapping_pages();
> 	...
> 
> AFAICT, this would be the most appropriate place to put it? Or, did
> you have something else in mind?

Yes, either this, or maybe in ext4_dio_write_iter() after returning from
ext4_buffered_write_iter() would be even more logical.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR