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Subject: Re: [PATCH 0/5] ext4: direct IO via iomap infrastructure
To:     Matthew Bobrowski <mbobrowski@mbobrowski.org>
Cc:     linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        jack@suse.cz, tytso@mit.edu
References: <cover.1565609891.git.mbobrowski@mbobrowski.org>
 <20190812173150.AF04F5204F@d06av21.portsmouth.uk.ibm.com>
 <20190813111004.GA12682@poseidon.bobrowski.net>
From:   RITESH HARJANI <riteshh@linux.ibm.com>
Date:   Tue, 13 Aug 2019 17:57:22 +0530
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On 8/13/19 4:40 PM, Matthew Bobrowski wrote:
> On Mon, Aug 12, 2019 at 11:01:50PM +0530, RITESH HARJANI wrote:
>>> This patch series converts the ext4 direct IO code paths to make use of the
>>> iomap infrastructure and removes the old buffer_head direct-io based
>>> implementation. The result is that ext4 is converted to the newer framework
>>> and that it may _possibly_ gain a performance boost for O_SYNC | O_DIRECT IO.
>>>
>>> These changes have been tested using xfstests in both DAX and non-DAX modes
>>> using various configurations i.e. 4k, dioread_nolock, dax.
>> I had some minor review comments posted on Patch-4.
>> But the rest of the patch series looks good to me.
> Thanks for the review, much appreciated! Also, apologies about any
> delayed response to your queries, I predominantly do all this work in
> my personal time.
Np at all.

>
>> I will also do some basic testing of xfstests which I did for my patches and
>> will revert back.
> Sounds good!

I did not find any failure new failures in xfstests with 4K block size.
Neither in basic fio DIO/AIO testing. So my basic testing looks good
(these are mostly the tests which I was using for my debug/dev too)


>
>> One query, could you please help answering below for my understanding :-
>>
>> I was under the assumption that we need to maintain
>> ext4_test_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN) or
>> atomic_read(&EXT4_I(inode)->i_unwritten))
>> in case of non-AIO directIO or AIO directIO case as well (when we may
>> allocate unwritten extents),
>> to protect with some kind of race with other parts of code(maybe
>> truncate/bufferedIO/fallocate not sure?) which may call for
>> ext4_can_extents_be_merged()
>> to check if extents can be merged or not.
>>
>> Is it not the case?
>> Now that directIO code has no way of specifying that this inode has
>> unwritten extent, will it not race with any other path, where this info was
>> necessary (like
>> in above func ext4_can_extents_be_merged())?
> Ah yes, I was under the same assumption when reviewing the code
> initially and one of my first solutions was to also use this dynamic
> 'state' flag in the ->end_io() handler. But, I fell flat on my face as
> that deemed to be problematic... This is because there can be multiple
> direct IOs to unwritten extents against the same inode, so you cannot
> possibly get away with tracking them using this single inode flag. So,
> hence the reason why we drop using EXT4_STATE_DIO_UNWRITTEN and use
> IOMAP_DIO_UNWRITTEN instead in the ->end_io() handler, which tracks
> whether _this_ particular IO has an underlying unwritten extent.

Thanks for taking time to explain this.

Yes, I do understand that part - i.e. while preparing block mapping in 
->iomap_begin
we get to know(from ext4_map_blocks) whether this is an unwritten extent 
and we add the flag
IOMAP_DIO_UNWRITTEN to iomap. This is needed so that we can convert 
unwritten extents in ->end_io
before we update the inode size and mark the inode dirty - to avoid any 
race with other AIO DIO or bufferedIO.

But what I meant was this (I may be wrong here since I haven't really 
looked into it),
but for my understanding I would like to discuss this -

So earlier with this flag(EXT4_STATE_DIO_UNWRITTEN) we were determining 
on whether a newextent can be merged with ex1 in function
ext4_extents_can_be_merged. But now since we have removed that flag we 
have no way of knowing that whether
this inode has any unwritten extents or not from any DIO path.
What I meant is isn't this removal of setting/unsetting of 
flag(EXT4_STATE_DIO_UNWRITTEN)
changing the behavior of this func - ext4_extents_can_be_merged?

Also - could you please explain why this check returns 0 in the first 
place (line 1762 - 1766 below)?

1733 int
1734 ext4_can_extents_be_merged(struct inode *inode, struct ext4_extent 
*ex1,
1735                                 struct ext4_extent *ex2)
<...>

1762         if (ext4_ext_is_unwritten(ex1) &&
1763             (ext4_test_inode_state(inode, EXT4_STATE_DIO_UNWRITTEN) ||
1764              atomic_read(&EXT4_I(inode)->i_unwritten) ||
1765              (ext1_ee_len + ext2_ee_len > EXT_UNWRITTEN_MAX_LEN)))
1766                 return 0;
<...>

Regards
Ritesh