Re: [PATCH V8 00/33] loop: Issue O_DIRECT aio using bio_vec

From: Dave Kleikamp
Date: Wed Aug 21 2013 - 13:12:51 EST


On 08/21/2013 11:39 AM, Benjamin LaHaise wrote:
> On Wed, Aug 21, 2013 at 11:30:22AM -0500, Dave Kleikamp wrote:
>> Ben,
>> First, let me apologize for neglecting to copy you and linux-aio on the
>> applicable patches. I've been carrying along this patchset, assuming I
>> had gotten the proper cc's correct a while back, but I somehow missed
>> the aio pieces.
>
> Thanks. Let's figure out how to tackle this best.
>
>> On 08/21/2013 08:02 AM, Benjamin LaHaise wrote:
> ...
>>> First off, have you tested that this series actually works when merged with
>>> the pending AIO changes from Kent? There a git tree with those pending
>>> changes at git://git.kvack.org/~bcrl/aio-next.git , and they're in
>>> linux-next.
>>
>> I've lightly tested the patchset against the linux-next tree, running a
>> fio job on loop-mounted filesystems of different fs types.
>
> Good to hear.
>>
>>> One of the major problems your changeset continues to carry is that your
>>> new read_iter/write_iter operations permit blocking (implicitely), which
>>> really isn't what we want for aio. If you're going to introduce a new api,
>>> it should be made non-blocking, and enforce that non-blocking requirement
>>> (ie warn when read_iter/write_iter methods perform blockin operations,
>>> similar to the warnings when scheduling in atomic mode). This means more
>>> changes for some filesystem code involved, something that people have been
>>> avoiding for years, but which really needs to be done.
>>
>> I'm not really sure how the read_iter and write_iter operations are more
>> likely to block than the current aio_read and aio_write operations. Am I
>> missing something?
>
> What you say is true, however, my point is more that it will be far easier
> to fix this issue by making it a hard constraint of a new API than it is
> to do a system-wide retrofit. You're converting code over to use the new
> API one by one, so adding a little bit more work to try and finally sort
> out this issue while making those conversions would be vrey helpful.
>
> I'm not saying that you should be required to write the code to cope with
> this additional requirement (I'm perfectly happy to help with that, and
> can probably get some time for that at $work), but more that if we're
> going to be changing all of the filesystems, we might as well try to get
> things right.

I don't really intend to make the patchset any more complicated than it
already is. The read/write_iter operations are intended to be as near a
replacement as possible to aio_read/write with the added ability to deal
with both kernel and user pages. A completely non-blocking interface
would be great, but that's a bit of work I'd rather not have to wait
for. Maybe that requirement can be added later.
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