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Date:	Tue, 4 Sep 2012 11:37:13 -0400 (EDT)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org, tytso@....edu, hughd@...gle.com,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/15 v2] jbd2: implement
 jbd2_journal_invalidatepage_range

On Tue, 4 Sep 2012, J. Bruce Fields wrote:

> Date: Tue, 4 Sep 2012 10:52:13 -0400
> From: J. Bruce Fields <bfields@...ldses.org>
> To: Lukas Czerner <lczerner@...hat.com>
> Cc: linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org, tytso@....edu,
>     hughd@...gle.com, linux-mm@...ck.org
> Subject: Re: [PATCH 02/15 v2] jbd2: implement
>     jbd2_journal_invalidatepage_range
> 
> On Fri, Aug 31, 2012 at 06:21:38PM -0400, Lukas Czerner wrote:
> > mm now supports invalidatepage_range address space operation and there
> > are two file system using jbd2 also implementing punch hole feature
> > which can benefit from this. We need to implement the same thing for
> > jbd2 layer in order to allow those file system take benefit of this
> > functionality.
> > 
> > With new function jbd2_journal_invalidatepage_range() we can now specify
> > length to invalidate, rather than assuming invalidate to the end of the
> > page.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> >  fs/jbd2/journal.c     |    1 +
> >  fs/jbd2/transaction.c |   19 +++++++++++++++++--
> >  include/linux/jbd2.h  |    2 ++
> >  3 files changed, 20 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> > index e149b99..e4618e9 100644
> > --- a/fs/jbd2/journal.c
> > +++ b/fs/jbd2/journal.c
> > @@ -86,6 +86,7 @@ EXPORT_SYMBOL(jbd2_journal_force_commit_nested);
> >  EXPORT_SYMBOL(jbd2_journal_wipe);
> >  EXPORT_SYMBOL(jbd2_journal_blocks_per_page);
> >  EXPORT_SYMBOL(jbd2_journal_invalidatepage);
> > +EXPORT_SYMBOL(jbd2_journal_invalidatepage_range);
> >  EXPORT_SYMBOL(jbd2_journal_try_to_free_buffers);
> >  EXPORT_SYMBOL(jbd2_journal_force_commit);
> >  EXPORT_SYMBOL(jbd2_journal_file_inode);
> > diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> > index fb1ab953..65c1374 100644
> > --- a/fs/jbd2/transaction.c
> > +++ b/fs/jbd2/transaction.c
> > @@ -1993,10 +1993,20 @@ zap_buffer_unlocked:
> >   *
> >   */
> >  void jbd2_journal_invalidatepage(journal_t *journal,
> > -		      struct page *page,
> > -		      unsigned long offset)
> > +				 struct page *page,
> > +				 unsigned long offset)
> > +{
> > +	jbd2_journal_invalidatepage_range(journal, page, offset,
> > +					  PAGE_CACHE_SIZE - offset);
> > +}
> > +
> > +void jbd2_journal_invalidatepage_range(journal_t *journal,
> > +				       struct page *page,
> > +				       unsigned int offset,
> > +				       unsigned int length)
> >  {
> >  	struct buffer_head *head, *bh, *next;
> > +	unsigned int stop = offset + length;
> >  	unsigned int curr_off = 0;
> >  	int may_free = 1;
> >  
> > @@ -2005,6 +2015,8 @@ void jbd2_journal_invalidatepage(journal_t *journal,
> >  	if (!page_has_buffers(page))
> >  		return;
> >  
> > +	BUG_ON(stop > PAGE_CACHE_SIZE || stop < length);
> 
> This misses e.g. length == (unsigned int)(-1), offset = 1.  Could make
> it obvious with:

Hmm.. So if length = -1 (e.g. UINT_MAX) and offset = 1 then:

offset + length = 0

so 

length is bigger than (offset + length) right ? Speaking in numbers:

length = 4294967295
offset = 1
stop = length + offset = 0

so (0 < 4294967295) is true and we'll BUG() on this, right ?

Am I missing something ?

-Lukas

> 
> 	BUG_ON(offset > PAGE_CACHE_SIZE || length > PAGE_CACHE_SIZE);
> 	BUG_ON(stop > PAGE_CACHE_SIZE);
> 
> Or is that overkill?
> 
> --b.
> 
> > +
> >  	/* We will potentially be playing with lists other than just the
> >  	 * data lists (especially for journaled data mode), so be
> >  	 * cautious in our locking. */
> > @@ -2014,6 +2026,9 @@ void jbd2_journal_invalidatepage(journal_t *journal,
> >  		unsigned int next_off = curr_off + bh->b_size;
> >  		next = bh->b_this_page;
> >  
> > +		if (next_off > stop)
> > +			return;
> > +
> >  		if (offset <= curr_off) {
> >  			/* This block is wholly outside the truncation point */
> >  			lock_buffer(bh);
> > diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> > index 3efc43f..21288fa 100644
> > --- a/include/linux/jbd2.h
> > +++ b/include/linux/jbd2.h
> > @@ -1101,6 +1101,8 @@ extern int	 jbd2_journal_forget (handle_t *, struct buffer_head *);
> >  extern void	 journal_sync_buffer (struct buffer_head *);
> >  extern void	 jbd2_journal_invalidatepage(journal_t *,
> >  				struct page *, unsigned long);
> > +extern void	 jbd2_journal_invalidatepage_range(journal_t *, struct page *,
> > +						   unsigned int, unsigned int);
> >  extern int	 jbd2_journal_try_to_free_buffers(journal_t *, struct page *, gfp_t);
> >  extern int	 jbd2_journal_stop(handle_t *);
> >  extern int	 jbd2_journal_flush (journal_t *);
> > -- 
> > 1.7.7.6
> > 
> > --
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> 
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