Re: mmap over nfs leads to excessive system load

[Andrew Morton]

Trond Myklebust <trond.myklebust@xxxxxxxxxx> wrote:
>
> On Wed, 2005-11-16 at 14:10 -0800, Andrew Morton wrote:
> 
> > > How is the filesystem supposed to distinguish between the cases
> > > "VM->writepage()", and "VM->writepages->mpage_writepages->writepage()"?
> > > 
> > 
> > Via the writeback_control, hopefully.
> > 
> > For write_one_page(), sync_mode==WB_SYNC_ALL, so NFS should start the I/O
> > immediately (it appears to not do so).
> 
> Sorry, but so does filemap_fdatawrite(). WB_SYNC_ALL clearly does not
> discriminate between a writepages() and a single writepage() situation,
> whatever the original intention was.

Could peek at wbc->nr_pages, or add another boolean to writeback_control
for this.

diff -puN include/linux/writeback.h~writeback_control-flag-writepages include/linux/writeback.h
--- devel/include/linux/writeback.h~writeback_control-flag-writepages	2005-11-16 14:43:52.000000000 -0800
+++ devel-akpm/include/linux/writeback.h	2005-11-16 14:43:52.000000000 -0800
@@ -53,10 +53,11 @@ struct writeback_control {
 	loff_t start;
 	loff_t end;
 
-	unsigned nonblocking:1;			/* Don't get stuck on request queues */
-	unsigned encountered_congestion:1;	/* An output: a queue is full */
-	unsigned for_kupdate:1;			/* A kupdate writeback */
-	unsigned for_reclaim:1;			/* Invoked from the page allocator */
+	unsigned nonblocking:1;		/* Don't get stuck on request queues */
+	unsigned encountered_congestion:1; /* An output: a queue is full */
+	unsigned for_kupdate:1;		/* A kupdate writeback */
+	unsigned for_reclaim:1;		/* Invoked from the page allocator */
+	unsigned for_writepages:1;	/* This is a writepages() call */
 };
 
 /*
diff -puN mm/page-writeback.c~writeback_control-flag-writepages mm/page-writeback.c
--- devel/mm/page-writeback.c~writeback_control-flag-writepages	2005-11-16 14:43:52.000000000 -0800
+++ devel-akpm/mm/page-writeback.c	2005-11-16 14:43:52.000000000 -0800
@@ -550,11 +550,17 @@ void __init page_writeback_init(void)
 
 int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
+	int ret;
+
 	if (wbc->nr_to_write <= 0)
 		return 0;
+	wbc->for_writepages = 1;
 	if (mapping->a_ops->writepages)
-		return mapping->a_ops->writepages(mapping, wbc);
-	return generic_writepages(mapping, wbc);
+		ret =  mapping->a_ops->writepages(mapping, wbc);
+	else
+		ret = generic_writepages(mapping, wbc);
+	wbc->for_writepages = 0;
+	return ret;
 }
 
 /**
_

> > For vmscan->writepage, wbc->for_reclaim is set, so we know that the IO
> > should be pushed immediately.  nfs_writepage() seems to dtrt here.
> > 
> > With the proposed changes, we don't need that iput() in nfs_writepage(). 
> > That worries me because I recall from a couple of years back that there are
> > really subtle races with doing iput() on the vmscan->writepage() path. 
> > Cannot remember what they were though...
> 
> Possibly to do with block filesystems that may trigger ->writepage()
> while inside iput_final()? NFS can't do that.

iput_final() can call truncate_inode_pages - maybe it was a deadlock, but
I'm fairly sure it was a race.
-
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