Re: why are WB_SYNC_NONE COMMITs being done with FLUSH_SYNC set ?

[Wu Fengguang <fengguang.wu@gmail.com>]

On Thu, Aug 19, 2010 at 10:37:10AM -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.

In principle all nonblocking checks in ->writepages should be removed.

(My original patch does have chunks for afs/cifs that somehow get
 dropped in the process, and missed ceph because it's not upstream
 when I started patch..)

> 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.
 
Since migration and pageout still set nonblocking for ->writepage, we
may keep them in the near future, until VM does not start IO on itself.

> > +	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.

Agreed.

Thanks,
Fengguang
---
writeback: remove useless nonblocking checks in ->writepages

This removes more deadcode that was somehow missed by commit 0d99519efef
(writeback: remove unused nonblocking and congestion checks).

The nonblocking checks in ->writepages are no longer used because the
flusher now prefer to block on get_request_wait() than to skip inodes on
IO congestion. The latter will lead to more seeky IO.

CC: Chris Mason <chris.mason@oracle.com>
CC: Jens Axboe <jens.axboe@oracle.com>
CC: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/afs/write.c |   16 +---------------
 fs/cifs/file.c |   10 ----------
 2 files changed, 1 insertion(+), 25 deletions(-)

--- linux-next.orig/fs/afs/write.c	2010-06-24 14:32:01.000000000 +0800
+++ linux-next/fs/afs/write.c	2010-08-20 07:03:01.000000000 +0800
@@ -455,8 +455,6 @@ int afs_writepage(struct page *page, str
 	}
 
 	wbc->nr_to_write -= ret;
-	if (wbc->nonblocking && bdi_write_congested(bdi))
-		wbc->encountered_congestion = 1;
 
 	_leave(" = 0");
 	return 0;
@@ -529,11 +527,6 @@ static int afs_writepages_region(struct 
 
 		wbc->nr_to_write -= ret;
 
-		if (wbc->nonblocking && bdi_write_congested(bdi)) {
-			wbc->encountered_congestion = 1;
-			break;
-		}
-
 		cond_resched();
 	} while (index < end && wbc->nr_to_write > 0);
 
@@ -554,18 +547,11 @@ int afs_writepages(struct address_space 
 
 	_enter("");
 
-	if (wbc->nonblocking && bdi_write_congested(bdi)) {
-		wbc->encountered_congestion = 1;
-		_leave(" = 0 [congest]");
-		return 0;
-	}
-
 	if (wbc->range_cyclic) {
 		start = mapping->writeback_index;
 		end = -1;
 		ret = afs_writepages_region(mapping, wbc, start, end, &next);
-		if (start > 0 && wbc->nr_to_write > 0 && ret == 0 &&
-		    !(wbc->nonblocking && wbc->encountered_congestion))
+		if (start > 0 && wbc->nr_to_write > 0 && ret == 0)
 			ret = afs_writepages_region(mapping, wbc, 0, start,
 						    &next);
 		mapping->writeback_index = next;
--- linux-next.orig/fs/cifs/file.c	2010-08-20 06:57:11.000000000 +0800
+++ linux-next/fs/cifs/file.c	2010-08-20 07:03:01.000000000 +0800
@@ -1379,16 +1379,6 @@ static int cifs_writepages(struct addres
 		return generic_writepages(mapping, wbc);
 
 
-	/*
-	 * BB: Is this meaningful for a non-block-device file system?
-	 * If it is, we should test it again after we do I/O
-	 */
-	if (wbc->nonblocking && bdi_write_congested(bdi)) {
-		wbc->encountered_congestion = 1;
-		kfree(iov);
-		return 0;
-	}
-
 	xid = GetXid();
 
 	pagevec_init(&pvec, 0);

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