linux-ext4 - Re: Checks in ext4_ext_fiemap

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Message-ID: <20110726121225.GC20131@quack.suse.cz>
Date:	Tue, 26 Jul 2011 14:12:25 +0200
From:	Jan Kara <jack@...e.cz>
To:	Yongqiang Yang <xiaoqiangnk@...il.com>
Cc:	Jan Kara <jack@...e.cz>, Ted Ts'o <tytso@....edu>,
	linux-ext4@...r.kernel.org, Andreas Dilger <adilger@...ger.ca>
Subject: Re: Checks in ext4_ext_fiemap_cb() broken

  Hi Yongqiang,

On Tue 26-07-11 09:20:28, Yongqiang Yang wrote:
> I have been thinking if we can handle fiemap much simpler for a while.
>  Current code is very ugly due to page cache look up.  I have a
> thought on simplifying these code.  The reason leading us to looking
> up page cache is that delayed extents are not in extents tree.  I
> think we can add an in-memory delayed extents list in inode, and we
> can delete entries in the list after we allocate blocks for them.
> There is no limit on length of extents in the list, this way can an
> entry contain as many blocks as they are contiguous logically.
> 
> What's your opinion?
  Yes, that should be doable and shouldn't have too big overhead. It's just
stupid we'll do all this stuff only for fiemap call which is relatively
rare.

								Honza

> On Mon, Jul 25, 2011 at 11:58 PM, Jan Kara <jack@...e.cz> wrote:
> >  Hello,
> >
> >  I just had a look at the code checking delayed allocated buffers in
> > ext4_ext_fiemap_cb(). I believe the checks there could use some elimiation
> > of common patterns but that's just a minor thing. The main problem is that
> > the code can easily crash the kernel when it races with page reclaim. You
> > just cannot access most of the page contents (and for buffers it is
> > especially true) without locking the page. Getting a reference via
> > find_get_pages_tag() guarantees you the structure cannot go away but mm is
> > still free to detach the page from the mapping at any moment. So you must
> > always lock a page and check that it still belongs to the desired mapping
> > before you check 'page_has_buffers()'.
> >
> >                                                                Honza
> > --
> > Jan Kara <jack@...e.cz>
> > SUSE Labs, CR
> >
> 
> 
> 
> -- 
> Best Wishes
> Yongqiang Yang
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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