Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:24967 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751425Ab0HSTQ3 (ORCPT ); Thu, 19 Aug 2010 15:16:29 -0400 Date: Thu, 19 Aug 2010 15:16:18 -0400 From: Jeff Layton To: Trond Myklebust Cc: Christoph Hellwig , fengguang.wu@gmail.com, linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org Subject: Re: why are WB_SYNC_NONE COMMITs being done with FLUSH_SYNC set ? Message-ID: <20100819151618.5f769dc9@tlielax.poochiereds.net> In-Reply-To: <1282229905.6199.19.camel@heimdal.trondhjem.org> References: <20100819101525.076831ad@barsoom.rdu.redhat.com> <20100819143710.GA4752@infradead.org> <1282229905.6199.19.camel@heimdal.trondhjem.org> Content-Type: text/plain; charset=US-ASCII Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Thu, 19 Aug 2010 10:58:25 -0400 Trond Myklebust 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 --- 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