Re: Async direct IO write vs buffered read race

[Rik van Riel <riel@redhat.com>]

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On Mon, 2017-06-26 at 11:11 -0400, Jeff Moyer wrote:
> Lukas Czerner <lczerner@redhat.com> writes:
> 
> > > The thing we do is a best effort thing that more or less
> > > guarantees that if
> > > you do say buffered IO and direct IO after that, it will work
> > > reasonably.
> > > However if direct and buffered IO can race, bad luck for your
> > > data. I don't
> > > think we want to sacrifice any performance of AIO DIO (and
> > > offloading of
> > > direct IO completion to a workqueue so that we can do
> > > invalidation costs
> > > noticeable mount of performance) for supporting such usecase.
> > 
> > What Jeff proposed would sacrifice performance for the case where
> > AIO
> > DIO write does race with buffered IO - the situation we agree is
> > not ideal
> > and should be avoided anyway. For the rest of AIO DIO this should
> > have no
> > effect right ? If true, I'd say this is a good effort to make sure
> > we do
> > not have disparity between page cache and disk.
> 
> Exactly.  Jan, are you concerned about impacting performance for
> mixed
> buffered I/O and direct writes?  If so, we could look into
> restricting
> the process context switch further, to just overlapping buffered and
> direct I/O (assuming there are no locking issues).
> 
> Alternatively, since we already know this is racy, we don't actually
> have to defer I/O completion to process context.  We could just
> complete
> the I/O as we normally would, but also queue up an
> invalidate_inode_pages2_range work item.  It will be asynchronous,
> but
> this is best effort, anyway.
> 
> As Eric mentioned, the thing that bothers me is that we have invalid
> data lingering in the page cache indefinitely.

Given that the requirement is that the page cache
gets invalidated after IO completion, would it be
possible to defer only the page cache invalidation
to task context, and handle the rest of the IO
completion in interrupt context?

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