On Mon, Dec 15, 2014 at 07:49:20AM -0500, Trond Myklebust wrote:

On Mon, Dec 15, 2014 at 12:26 AM, Omar Sandoval [off-list ref] wrote:

quoted

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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>