Re: [PATCH] ext4: Return the length of a hole from get_block

[Jan Kara <jack@suse.cz>]

On Mon 13-07-15 11:26:15, Matthew Wilcox wrote:
> On Mon, Jul 13, 2015 at 05:16:10PM +0200, Jan Kara wrote:
> > On Fri 03-07-15 11:15:11, Matthew Wilcox wrote:
> > > From: Matthew Wilcox <willy@linux.intel.com>
> > > 
> > > Currently, if ext4's get_block encounters a hole, it does not modify the
> > > buffer_head.  That's fine for many callers, but for DAX, it's useful to
> > > know how large the hole is.  XFS already returns the length of the hole,
> > > so this improvement should not confuse any callers.
> > > 
> > > Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
> > 
> > So I'm somewhat wondering: What is the reason of BH_Uptodate flag being
> > set? I can see the XFS sets it in some cases as well but the use of the
> > flag isn't really clear to me...
> 
> No clue.  I'm just following the documentation in buffer.c:
> 
>  * NOTE! All mapped/uptodate combinations are valid:
>  *
>  *      Mapped  Uptodate        Meaning
>  *
>  *      No      No              "unknown" - must do get_block()
>  *      No      Yes             "hole" - zero-filled
>  *      Yes     No              "allocated" - allocated on disk, not read in
>  *      Yes     Yes             "valid" - allocated and up-to-date in memory.

OK, but that speaks about buffer head attached to a page. get_block()
callback gets a temporary bh (at least in some cases) only so that it can
communicate result of block mapping. And BH_Uptodate should be set only if
data in the buffer is properly filled (which cannot be the case for
temporary bh which doesn't have *any* data) and it simply isn't the case
even for bh attached to a page because ext4 get_block() functions don't
touch bh->b_data at all. So I just wouldn't set BH_Uptodate in get_block()
at all..

									Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR