Re: [RFC] fs io with struct page instead of iovecs

[Zach Brown <zach.brown@oracle.com>]

Badari Pulavarty wrote:
> On Tue, 2007-11-06 at 17:43 -0800, Zach Brown wrote:
>> At the FS meeting at LCE there was some talk of doing O_DIRECT writes from the
>> kernel with pages instead of with iovecs.  T
> 
> Why ? Whats the use case ?

Well, I think there's a few:

There are existing callers which hold a kmap() across ->write, which
isn't great.  ecryptfs() does this.  That's mentioned in the patch
series.  Arguably loopback should be using this instead of copying some
fs paths and trying to call aop methods directly.

I seem to remember Christoph and David having stories of knfsd folks in
SGI wanting to do O_DIRECT writes from knfsd?  (If not, *I* kind of want
to, after rolling some patches to align net rx descriptors :)).

Lustre shows us that there is a point at which you can't saturate your
network and storage if your cpu is copying all the data.  I'll be the
first to admit that the community might not feel a pressing need to
address this for in-kernel file system writers, but the observation remains.

- z