linux-ext4 - Re: [PATCH] ext4: Return the length of a hole from get

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Message-ID: <20150715095903.GE22609@quack.suse.cz>
Date:	Wed, 15 Jul 2015 11:59:03 +0200
From:	Jan Kara <jack@...e.cz>
To:	Matthew Wilcox <willy@...ux.intel.com>
Cc:	Jan Kara <jack@...e.cz>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH] ext4: Return the length of a hole from get_block

On Tue 14-07-15 09:48:51, Matthew Wilcox wrote:
> On Tue, Jul 14, 2015 at 11:02:46AM +0200, Jan Kara wrote:
> > 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@...ux.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@...ux.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..
> 
> OK, but how should DAX then distinguish between an old-style filesystem
> (like current ext4) which reports "unknown" and leaves b_size untouched
> when it encounters a hole, versus a new-style filesystem (XFS, ext4 with
> this patch) which wants to report the size of a hole in b_size?  The use
> of Uptodate currently distinguishes the two cases.
> 
> Plus, why would you want bh's to be treated differently, depending on
> whether they're stack-based or attached to a page?  That seems even more
> confusing than bh's already are.

Well, you may want to treat them differently because they *are* different.
For example touching b_size of page-attached buffer_head is a no-go.
get_block() interface is abusing buffer_head structure for historical
reasons.

Seeing you have hit issues with using buffer_head for passing mapping
information I agree with Dave that we should convert DAX code to use
iomaps instead of cluttering get_block() via buffer_head further. You can
lift struct iomap from include/linux/exportfs.h (and related constant
definitions) and use it for passing map information. It should be quite
straightforward and simple now that DAX doesn't have many users. We will
have:

typedef int (iomap_fn_t)(struct inode *inode, loff_t offset, u64 length,
			 bool create, struct iomap *iomap);

and DAX functions will take this instead of get_block_t. Adding a wrapper
to ext4_map_blocks() to work as iomap_fn_t is pretty straightforward as
well. I'm sorry we didn't come up with this immediately when you started
implementing DAX...

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