Re: [PATCH 2/9] cachefiles: drop direct usage of ->bmap method.

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On Fri, Aug 30, 2019 at 05:17:51PM +0100, David Howells wrote:
> Christoph Hellwig <hch@xxxxxx> wrote:
> 
> > Not related to this patch, but using iov_iter with dio is trivial, what
> > is the blocker therere?
> 
> The usual: time.
> 
> The change as a whole is not actually trivial since it will involve completely
> overhauling the fscache data API and how the filesystems use it - and then
> having cachefiles perform the DIO asynchronously as per Trond's requirements
> for using fscache with NFS.

Well, doing in-kernel async I/O is actually rather trivial these days.
Take a look at the loop driver for an example.

> I also need to work out how I'm going to do data/hole detection.  Can I set,
> say, O_NOREADHOLE and then expect the DIO to stop early with a short read?  Or
> do I need to use SEEK_DATA/SEEK_HOLE in advance to define the occupied
> regions?

We'll you'd need to implement a IOCB_NOHOLE, but that wouldn't be all
that hard.  Having a READ_PLUS like interface that actually tells you
how large the hole is might be hard.



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