Re: [PATCH 1/8] nfs: follow direct I/O write locking convention

[Omar Sandoval <osandov@osandov.com>]

On Mon, Dec 15, 2014 at 07:49:20AM -0500, Trond Myklebust wrote:
> On Mon, Dec 15, 2014 at 12:26 AM, Omar Sandoval <osandov@osandov.com> wrote:
> > The generic callers of direct_IO lock i_mutex before doing a write. NFS
> > doesn't use the generic write code, so it doesn't follow this
> > convention. This is now a problem because the interface introduced for
> > swap-over-NFS calls direct_IO for a write without holding i_mutex, but
> > other implementations of direct_IO will expect to have it locked.
> 
> I really don't care much about swap-over-NFS performance; that's a
> niche usage at best. I _do_ care about O_DIRECT performance, and the
> ability to run multiple WRITE calls in parallel.
> 
> IOW: Patch NACKed... Please find another solution.
> 
> Trond

So the patch formatting doesn't make it completely clear what's going on
here, but here's what the original nfs_file_direct_write code did:

- called with i_mutex unlocked
- collects stats and does some generic checks
- locks i_mutex
- syncs the mapping, schedules the write
- unlocks i_mutex
- waits for the write to complete if synchronous

After this patch, nfs_file_direct_write works like:

- called with i_mutex locked
- collects stats and does some generic checks
- syncs the mapping, schedules the write
- drops i_mutex
- waits for the write to complete if synchronous
- picks i_mutex back up

There's an extra lock and unlock as a result and a slightly longer
critical section, but we drop i_mutex to wait for the write, so multiple
writes still work in parallel.

-- 
Omar