linux-kernel - Re: [vm] writing to UDF DVD+RW (/dev/sr0) while under memory pressure: box ==> doorstop

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Message-Id: <1199877080.4340.19.camel@homer.simson.net>
Date:	Wed, 09 Jan 2008 12:11:20 +0100
From:	Mike Galbraith <efault@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: [vm] writing to UDF DVD+RW (/dev/sr0) while under memory
	pressure: box ==> doorstop


On Tue, 2008-01-08 at 16:27 +0100, Mike Galbraith wrote:
> On Tue, 2008-01-08 at 16:21 +0100, Mike Galbraith wrote:
> > On Tue, 2008-01-08 at 03:38 -0800, Andrew Morton wrote:
> > > 
> > > Well.  From your earlier trace it appeared that something was causing
> > > the filesystem to perform synchronous inode writes - sync_dirty_buffer() was
> > > called.
> > > 
> > > This will cause many more seeks than would occur if we were doing full
> > > delayed writing, with obvious throughput implications.
> > 
> > Yes, with UDF, the IO was _incredibly_ slow.  With ext2, it was better,
> > though still very bad.  I tested with that other OS, and it gets ~same
> > throughput with UDF as I got with ext2 (ick).
> > 
> > UDF does udf_clear_inode() -> write_inode_now(inode, 1)
> > 
> > I suppose I could try write_inode_now(inode, 0).  Might unstick the box.
> 
> (nope, still sync, UDF still deadly)

write_inode_now() is a fibber.

The below seems to fix it in that writes dribbling to the DVD+RW at the
whopping 1 to 10 pages/sec I'm seeing no longer turn box into a
doorstop.  It's probably busted as heck.

Think I'll cc linux-mm, and go find something safer to play with.

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 0fca820..f1cce24 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -657,7 +657,7 @@ int write_inode_now(struct inode *inode, int sync)
 	int ret;
 	struct writeback_control wbc = {
 		.nr_to_write = LONG_MAX,
-		.sync_mode = WB_SYNC_ALL,
+		.sync_mode = sync ? WB_SYNC_ALL : WB_SYNC_NONE,
 		.range_start = 0,
 		.range_end = LLONG_MAX,
 	};
diff --git a/fs/udf/inode.c b/fs/udf/inode.c
index 6ff8151..d1fc116 100644
--- a/fs/udf/inode.c
+++ b/fs/udf/inode.c
@@ -117,7 +117,7 @@ void udf_clear_inode(struct inode *inode)
 		udf_discard_prealloc(inode);
 		udf_truncate_tail_extent(inode);
 		unlock_kernel();
-		write_inode_now(inode, 1);
+		write_inode_now(inode, 0);
 	}
 	kfree(UDF_I_DATA(inode));
 	UDF_I_DATA(inode) = NULL;


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