From: Jan Kara <jack@suse.cz>
To: Yongqiang Yang <xiaoqiangnk@gmail.com>
Cc: Jan Kara <jack@suse.cz>, Ted Ts'o <tytso@mit.edu>,
linux-ext4@vger.kernel.org, Andreas Dilger <adilger@dilger.ca>
Subject: Re: Checks in ext4_ext_fiemap_cb() broken
Date: Tue, 26 Jul 2011 14:12:25 +0200 [thread overview]
Message-ID: <20110726121225.GC20131@quack.suse.cz> (raw)
In-Reply-To: <CAGBYx2ZsxvmEcC0akMRQTgDWUmkUHOp=D4Ff69_pSMb+bGH5Sg@mail.gmail.com>
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@suse.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@suse.cz>
> > SUSE Labs, CR
> >
>
>
>
> --
> Best Wishes
> Yongqiang Yang
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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next prev parent reply other threads:[~2011-07-26 12:12 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-25 15:58 Checks in ext4_ext_fiemap_cb() broken Jan Kara
2011-07-26 1:20 ` Yongqiang Yang
2011-07-26 12:12 ` Jan Kara [this message]
2011-07-26 12:48 ` Yongqiang Yang
2011-07-26 16:30 ` Allison Henderson
2011-07-26 16:44 ` Andreas Dilger
2011-07-26 17:07 ` Ted Ts'o
2011-07-26 18:48 ` Aditya Kali
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