linux-kernel - Re: Linux 2.6.29

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Date:	Fri, 27 Mar 2009 08:57:23 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Jeff Garzik <jeff@...zik.org>, Theodore Tso <tytso@....edu>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Wed, Mar 25 2009, Linus Torvalds wrote:
> 
> 
> On Wed, 25 Mar 2009, Jeff Garzik wrote:
> > 
> > It is clearly possible to implement an fsync(2) that causes FLUSH CACHE to be
> > issued, without adding full barrier support to a filesystem.  It is likely
> > doable to avoid touching per-filesystem code at all, if we issue the flush
> > from a generic fsync(2) code path in the kernel.
> 
> We could easily do that. It would even work for most cases. The 
> problematic ones are where filesystems do their own disk management, but I 
> guess those people can do their own fsync() management too.
> 
> Somebody send me the patch, we can try it out.

Here's a simple patch that does that. Not even tested, it compiles. Note
that file systems that currently do blkdev_issue_flush() in their
->sync() should then get it removed.

> > Remember, fsync(2) means that the user _expects_ a performance hit.
> 
> Within reason, though.
> 
> OS X, for example, doesn't do the disk barrier. It requires you to do a 
> separate FULL_FSYNC (or something similar) ioctl to get that. Apparently 
> exactly because users don't expect quite _that_ big of a performance hit.
> 
> (Or maybe just because it was easier to do that way. Never attribute to 
> malice what can be sufficiently explained by stupidity).

It'd be better to have a knob to control whether fsync() should care
about the hardware side as well, instead of trying to teach applications
to use FULL_FSYNC.

diff --git a/fs/sync.c b/fs/sync.c
index ec95a69..7a44d4e 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -8,6 +8,7 @@
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/writeback.h>
+#include <linux/blkdev.h>
 #include <linux/syscalls.h>
 #include <linux/linkage.h>
 #include <linux/pagemap.h>
@@ -104,6 +105,7 @@ int vfs_fsync(struct file *file, struct dentry *dentry, int datasync)
 {
 	const struct file_operations *fop;
 	struct address_space *mapping;
+	struct block_device *bdev;
 	int err, ret;
 
 	/*
@@ -138,6 +140,13 @@ int vfs_fsync(struct file *file, struct dentry *dentry, int datasync)
 	err = filemap_fdatawait(mapping);
 	if (!ret)
 		ret = err;
+
+	bdev = mapping->host->i_sb->s_bdev;
+	if (bdev) {
+		err = blkdev_issue_flush(bdev, NULL);
+		if (!ret)
+			ret = err;
+	}
 out:
 	return ret;
 }

-- 
Jens Axboe

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