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
