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Subject: Re: [PATCH 0/5] ext4: direct IO via iomap infrastructure
To:     Matthew Bobrowski <mbobrowski@mbobrowski.org>, tytso@mit.edu,
        jack@suse.cz, adilger.kernel@dilger.ca
Cc:     linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        hch@infradead.org, aneesh.kumar@linux.ibm.com
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From:   Ritesh Harjani <riteshh@linux.ibm.com>
Date:   Thu, 22 Aug 2019 19:41:25 +0530
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On 8/22/19 5:30 PM, Matthew Bobrowski wrote:
> On Wed, Aug 21, 2019 at 11:14:07PM +1000, Matthew Bobrowski wrote:
>> On Tue, Aug 13, 2019 at 05:57:22PM +0530, RITESH HARJANI wrote:
>>> 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?
>>
>> OK, I'm stuck and looking for either clarity, revalidation of my
>> thought process, or any input on how to solve this problem for that
>> matter.
>>
>> In the current ext4 direct IO implementation, the dynamic state flag
>> EXT4_STATE_DIO_UNWRITTEN is set/unset for synchronous direct IO
>> writes. On the other hand, the flag EXT4_IO_END_UNWRITTEN is set/unset
>> for ext4_io_end->flag, and the value of i_unwritten is
>> incremented/decremented for asynchronous direct IO writes. All
>> mechanisms by which are used to track and determine whether the inode,
>> or an IO in flight against a particular inode have any pending
>> unwritten extents that need to be converted after the IO has
>> completed. In addition to this, we have ext4_can_extents_be_merged()
>> performing explicit checks against both EXT4_STATE_DIO_UNWRITTEN and
>> i_unwritten and using them to determine whether it can or cannot merge
>> a requested extent into an existing extent.
>>
>> This is all fine for the current direct IO implementation. However,
>> while switching the direct IO code paths over to make use of the iomap
>> infrastructure, I believe that we can no longer simply track whether
>> an inode has unwritten extents needing to be converted by simply
>> setting and checking the EXT4_STATE_DIO_UNWRITTEN flag on the
>> inode. The reason being is that there can be multiple direct IO
>> operations to unwritten extents running against the inode and we don't
>> particularly distinguish synchronous from asynchronous writes within
>> ext4_iomap_begin() as there's really no need. So, the only way to
>> accurately determine whether extent conversion is deemed necessary for
>> an IO operation whether it'd be synchronous or asynchronous is by
>> checking the IOMAP_DIO_UNWRITTEN flag within the ->end_io()
>> callback. I'm certain that this portion of the logic is correct, but
>> we're still left with some issues when it comes to the checks that I
>> previously mentioned in ext4_can_extents_be_merged(), which is the
>> part I need some input on.
>>
>> I was doing some thinking and I don't believe that making use of the
>> EXT4_STATE_DIO_UNWRITTEN flag is the solution at all here. This is not
>> only for reasons that I've briefly mentioned above, but also because
>> of the fact that it'll probably lead to a lot of inaccurate judgements
>> while taking particular code paths and some really ugly code that
>> creeps close to the definition of insanity. Rather, what if we solve
>> this problem by continuing to just use i_unwritten to keep track of
>> all the direct IOs to unwritten against running against an inode?
>> Within ext4_iomap_begin() post successful creation of unwritten
>> extents we'd call atomic_inc(&EXT4_I(inode)->i_unwritten) and
>> subsequently within the ->end_io() callback whether we take the
>> success or error path we'd call
>> atomic_dec(&EXT4_I(inode)->i_unwritten) accordingly? This way we can
>> still rely on this value to be used in the check within
>> ext4_can_extents_be_merged(). Open for alternate suggestions if anyone
>> has any...
> 
> Actually, no...
> 
> I've done some more thinking and what I suggested above around the use
> of i_unwritten will also not work properly. Using iomap
> infrastructure, there is the possibility of calling into the
> ->iomap_begin() more than once for a single direct IO operation. This
> means that by the time we even get to decrementing i_unwritten in the
> ->end_io() callback after converting the unwritten extents we're
> already running the possibility of i_unwritten becoming unbalanced
> really quickly and staying that way. This also means that the
> statement checking i_unwritten in ext4_can_extents_be_merged() will be
> affected and potentially result in it being evaluated incorrectly. I
> was thinking that we could just decrement i_unwritten in
> ->iomap_end(), but that seems to me like it would be racy and also
> lead to incorrect results. At this point I'm out of ideas on how to
> solve this, so any other ideas would be appreciated!

I will let others also comment, if someone has any other better approach.

1. One approach is to add the infrastructure in iomap with 
iomap_dio->private which is filesystem specific pointer. This can be
updated by filesystem in ->iomap_begin call into iomap->private.
And in case of iomap_dio_rw, this iomap->private will be copied to 
iomap_dio->private if not already set.

But I think this will eventually become hacky in the sense when you will 
have to determine on whether the dio->private is already set or not when 
iomap_apply will be called second time. It will become a problem with 
AIO DIO in ext4 since we use i_unwritten which tells us whether there is 
any unwritten extent but it does not tell whether this unwritten extent 
is due to a DIRECT AIO DIO in progress or a buffered one.

So we can ignore this approach - unless you or someone else have some 
good design ideas to build on top of above.


2. Second approach which I was thinking is to track only those extents 
which are marked unwritten and are under IO. This can be done in 
ext4_map_blocks. This way we will not have to track a particular inode 
has any unwritten extents or not, but it will be extent based.
Something similar was also done a while ago. Do you think this approach 
will work in our case?

So with this extents will be scanned in extent status tree to see if any 
among those are under IO and are unwritten in func 
ext4_can_extents_be_merged.

https://patchwork.ozlabs.org/patch/1013837/


-Ritesh