Re: Linux 2.6.29

Jens Axboe
2009-03-27T07:57:36Z

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