linux-kernel - Re: [RFC] block integrity: Fix write after checksum calculation problem

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Message-Id: <1302543595-sup-4352@think>
Date:	Mon, 11 Apr 2011 13:41:16 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	djwong <djwong@...ibm.com>, Jan Kara <jack@...e.cz>,
	Dave Chinner <david@...morbit.com>,
	Joel Becker <jlbec@...lplan.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Jens Axboe <axboe@...nel.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Mingming Cao <mcao@...ibm.com>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [RFC] block integrity: Fix write after checksum calculation problem

Excerpts from Jeff Layton's message of 2011-04-11 12:42:29 -0400:
> > @@ -5839,6 +5844,15 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> >      if (ret < 0)
> >          goto out_unlock;
> >      ret = 0;
> > +
> > +    /*
> > +     * write_begin/end might have created a dirty page and someone
> > +     * could wander in and start the IO.  Make sure that hasn't
> > +     * happened.
> > +     */
> > +    lock_page(page);
> > +    wait_on_page_writeback(page);
> > +    unlock_page(page);
> 
> nit:
> 
> The callers of page_mkwrite always lock the page afterward if you
> return from page_mkwrite with it unlocked. If you plan to take page
> lock anyway, it's probably slightly more efficient not to unlock it and
> instead return VM_FAULT_LOCKED.
> 

Actually this isn't a nit.  Keeping the page locked closes an important
hole where it can become writeback again.  It might fix the last
remaining problem.

-chris
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