linux-kernel - Re: [PATCH 2/9] vfs: export do_splice

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Message-ID: <20130320221902.GA28432@quack.suse.cz>
Date:	Wed, 20 Mar 2013 23:19:02 +0100
From:	Jan Kara <jack@...e.cz>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Jan Kara <jack@...e.cz>, David Howells <dhowells@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	sedat.dilek@...glemail.com, hooanon05@...oo.co.jp, mszeredi@...e.cz
Subject: Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

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;
        smp_wmb();
        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.).
	sync_filesystem(sb);
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
done.
 
> 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.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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