On Thu, 19 Aug 2010 10:58:25 -0400
Trond Myklebust <trond.myklebust@xxxxxxxxxx> wrote:
> On Thu, 2010-08-19 at 10:37 -0400, Christoph Hellwig wrote:
> > On Thu, Aug 19, 2010 at 10:15:25AM -0400, Jeff Layton wrote:
> > > I'm looking at backporting some upstream changes to earlier kernels,
> > > and ran across something I don't quite understand...
> > >
> > > In nfs_commit_unstable_pages, we set the flags to FLUSH_SYNC. We then
> > > zero out the flags if wbc->nonblocking or wbc->for_background is set.
> > >
> > > Shouldn't we also clear it out if wbc->sync_mode == WB_SYNC_NONE ?
> > > WB_SYNC_NONE means "don't wait on anything", so shouldn't that include
> > > not waiting on the COMMIT to complete?
> >
> > I've been trying to figure out what the nonblocking flag is supposed
> > to mean for a while now.
> >
> > It basically disappeared in commit 0d99519efef15fd0cf84a849492c7b1deee1e4b7
> >
> > "writeback: remove unused nonblocking and congestion checks"
> >
> > from Wu. What's left these days is a couple of places in local copies
> > of write_cache_pages (afs, cifs), and a couple of checks in random
> > writepages instances (afs, block_write_full_page, ceph, nfs, reiserfs, xfs)
> > and the use in nfs_write_inode. It's only actually set for memory
> > migration and pageout, that is VM writeback.
> >
> > To me it really doesn't make much sense, but maybe someone has a better
> > idea what it is for.
> >
> > > + if (wbc->nonblocking || wbc->for_background ||
> > > + wbc->sync_mode == WB_SYNC_NONE)
> >
> > You could remove the nonblocking and for_background checks as
> > these impliy WB_SYNC_NONE.
>
> To me that sounds fine. I've also been trying to wrap my head around the
> differences between 'nonblocking', 'for_background', 'for_reclaim' and
> 'for_kupdate' and how the filesystem is supposed to treat them.
>
> Aside from the above, I've used 'for_reclaim', 'for_kupdate' and
> 'for_background' in order to adjust the RPC request's queuing priority
> (high in the case of 'for_reclaim' and low for the other two).
>
Here's a lightly tested patch that turns the check for the two flags
into a check for WB_SYNC_NONE. It seems to do the right thing, but I
don't have a clear testcase for it. Does this look reasonable?
------------------[snip]------------------------
NFS: don't use FLUSH_SYNC on WB_SYNC_NONE COMMIT calls
WB_SYNC_NONE is supposed to mean "don't wait on anything". That should
also include not waiting for COMMIT calls to complete.
WB_SYNC_NONE is also implied when wbc->nonblocking or
wbc->for_background are set, so we can replace those checks in
nfs_commit_unstable_pages with a check for WB_SYNC_NONE.
Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
fs/nfs/write.c | 10 +++++-----
1 files changed, 5 insertions(+), 5 deletions(-)
diff --git a/fs/nfs/write.c b/fs/nfs/write.c
index 874972d..35bd7d0 100644
--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1436,12 +1436,12 @@ static int nfs_commit_unstable_pages(struct inode *inode, struct writeback_contr
/* Don't commit yet if this is a non-blocking flush and there are
* lots of outstanding writes for this mapping.
*/
- if (wbc->sync_mode == WB_SYNC_NONE &&
- nfsi->ncommit <= (nfsi->npages >> 1))
- goto out_mark_dirty;
-
- if (wbc->nonblocking || wbc->for_background)
+ if (wbc->sync_mode == WB_SYNC_NONE) {
+ if (nfsi->ncommit <= (nfsi->npages >> 1))
+ goto out_mark_dirty;
flags = 0;
+ }
+
ret = nfs_commit_inode(inode, flags);
if (ret >= 0) {
if (wbc->sync_mode == WB_SYNC_NONE) {
--
1.5.5.6
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