Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

From: Jan Kara
Date: Wed Mar 20 2013 - 18:19:11 EST

On Wed 20-03-13 21:48:13, Al Viro wrote:
> On Wed, Mar 20, 2013 at 08:52:22PM +0100, Jan Kara wrote:
> > > do_bio_filebacked(), with some ugliness between that and callsite. Note,
> > > BTW, that we have a pair of possible vfs_fsync() calls in there; how do those
> > > interact with freeze?
> > Freezing code takes care that all dirty data is synced before fs is
> > frozen and no new dirty data can be created before fs is thawed. So
> > vfs_fsync() should just return without doing anything on frozen filesystem.
> Um... How does it interact with vfs_fsync() already in progress when you
> ask to freeze it?
So the exact sequence of freezing is:
sb->s_writers.frozen = SB_FREEZE_WRITE;
sb_wait_write(sb, SB_FREEZE_WRITE);
Now there are no processes in sb_start_write() - sb_end_write() section.
Then we do the same for SB_FREEZE_PAGEFAULT. After this noone should be
able to dirty a page or inode. Writeback or vfs_fsync() may be still
running (so fs can be creating new transactions in the journal for
writeback etc.).
After this there should be no dirty data so although we can still be
somewhere inside vfs_fsync() it should have nothing to do.

Now we freeze to state SB_FREEZE_FS (nop for ext4, but for XFS it may
interact e.g. with inode reclaim trimming preallocated blocks) and we are

> Anyway, I've pulled the fscker out of ->aio_write, ->write and ->splice_write;
> on that pathway it's in the do_splice_from() (see vfs.git#experimental).
> ... and now, for something *really* nasty: where do mandatory file locks
> belong in the locking hierarchy? Relative to fsfreeze one, for starters,
> but both for unionmount and overlayfs we need to decide where they live
> relative to ->i_mutex on directories.
Hum, interesting question :). Relative to fsfreeze, it doesn't seem to
matter much, does it? We don't actually lock / unlock these from write
paths needing freeze protection, we only wait for them. But maybe I miss
some ugly case you have in mind.

Jan Kara <jack@xxxxxxx>
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at