Re: [PATCH v2] writeback: Do not sync data dirtied after sync start

[Fengguang Wu <fengguang.wu@intel.com>]

On Mon, Sep 30, 2013 at 11:31:49AM +0200, Jan Kara wrote:
> On Sat 28-09-13 08:31:19, Wu Fengguang wrote:
> > Hi Jan,
> > 
> > On Thu, Sep 26, 2013 at 09:23:58PM +0200, Jan Kara wrote:
> > > When there are processes heavily creating small files while sync(2) is
> > > running, it can easily happen that quite some new files are created
> > > between WB_SYNC_NONE and WB_SYNC_ALL pass of sync(2). That can happen
> > > especially if there are several busy filesystems (remember that sync
> > > traverses filesystems sequentially and waits in WB_SYNC_ALL phase on one
> > > fs before starting it on another fs). Because WB_SYNC_ALL pass is slow
> > > (e.g. causes a transaction commit and cache flush for each inode in
> > > ext3), resulting sync(2) times are rather large.
> > 
> > This is a very good change. An old problem that may worth noting here
> > is that inode_dirtied_after() has a workaround for the inodes whose
> > ->dirtied_when is never updated due to being constantly redirtied.
> > That workaround still leaves a small time window that sync() may skip
> > a should-be-synced inode. Since the problem existed before this patch
> > so I'm fine with this change.
>   Thanks for review. Do you mean the situation when jiffies in
> inode->dirtied_when essentially wrap around on 32-bit systems wrt current
> time?

Yes.

> Yes, that is still a problem for which I don't know a better fix than
> the current workaround.

OK, that's fine. I don't have ideas on this, either.

Thanks,
Fengguang