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Subject: Re: Is ext3 flush data to disk when files are closed?
From: Badari Pulavarty <pbadari@gmail.com>
To: Andrew Morton <akpm@osdl.org>
Cc: uszhaoxin@gmail.com, lkml <linux-kernel@vger.kernel.org>
In-Reply-To: <20051019150940.477e2184.akpm@osdl.org>
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Content-Type: text/plain
Date: Wed, 19 Oct 2005 15:16:39 -0700
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On Wed, 2005-10-19 at 15:09 -0700, Andrew Morton wrote:
> Badari Pulavarty <pbadari@gmail.com> wrote:
> >
> > On Wed, 2005-10-19 at 13:00 -0700, Andrew Morton wrote:
> > > Badari Pulavarty <pbadari@gmail.com> wrote:
> > > >
> > > > On Wed, 2005-10-19 at 11:31 -0400, Xin Zhao wrote:
> > > > > As far as I know, if an application modifies a file on an ext3 file
> > > > > ssytem, it actually change the page cache, and the dirty pages will be
> > > > > flushed to disk by kupdate periodically.
> > > > > 
> > > > > My question is: if a file is to be closed, but some of its data pages
> > > > > are marked as dirty, will system block on close() and wait for dirty
> > > > > pages being flushed to disk? If so, it seems to decrease performance
> > > > > significantly if a lot of updates on many small files are involved.
> > > > > 
> > > > > Can someone point me to the right place to check how it works? Thanks!
> > > > 
> > > > On the last close() of the file, it should be flushed through ..
> > > > 
> > > > 	iput_final() -> generic_drop_inode() -> write_inode_now()
> > > > 		-> __writeback_single_inode() -> __sync_single_inode()
> > > > 			-> do_writepages()
> > > 
> > > The dcache's reference to the inode will prevent this from happening at
> > > close() time.
> > > 
> > 
> > I thought so too, till I wrote a kprobe/systemtap script to print 
> > the callers of generic_forget_inode() earlier and saw that most 
> > of my stacks are from exit() or close().
> > 
> >  0xffffffff801a0222 : generic_drop_inode+0x2/0x170 []
> >  0xffffffff8019eeb0 : iput+0x50/0x90 []
> >  0xffffffff8019c7bb : dput+0x1db/0x220 []
> >  0xffffffff80184461 : __fput+0x171/0x1e0 []
> >  0xffffffff801829ce : filp_close+0x6e/0x90 []
> >  0xffffffff801388eb : put_files_struct+0x6b/0xc0 []
> >  0xffffffff801392ef : do_exit+0x1ff/0xbb0 []
> >  
> 
> But generic_forget_inode usually doesn't dispose of the inode.
> 
> 	if (!hlist_unhashed(&inode->i_hash)) {
> 		if (!(inode->i_state & (I_DIRTY|I_LOCK)))
> 			list_move(&inode->i_list, &inode_unused);
> 		inodes_stat.nr_unused++;
> 		if (!sb || (sb->s_flags & MS_ACTIVE)) {
> 			spin_unlock(&inode_lock);
> 			return;
> 		}

Okay, makes sense. 

Thanks,
Badari

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