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

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Date:	Mon, 11 Apr 2011 17:46:52 -0700
From:	Mingming Cao <cmm@...ibm.com>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Jeff Layton <jlayton@...hat.com>, 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

On Mon, 2011-04-11 at 13:41 -0400, Chris Mason wrote:
> 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);
> > 

I am little puzzled here. if someone wander in and start the IO, the
page is up-to-date (dirtied by this page_mkwrite). We shouldn't see the
checksum inconsistancy, right?

> > 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.
> 

Oh, right. Currently ext4_page_mkwrite drops the page lock before
calling it's dirty the page (by write_begin() and write_end().  I
suspect regrab the lock() after write_end() (with your proposed change)
and returning with locked still leave the dirty by ext4_page_mkwrite
unlocked.  We probably should to keep the page locked the page during
the entire ext4_page_mkwrite() call. Any reason to drop the page lock()
before calling aops->write_begin()?

Mingming

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