Re: [PATCH 0/5] ext4: direct IO via iomap infrastructure

[Matthew Bobrowski <mbobrowski@mbobrowski.org>]

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!

--M