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

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Message-ID: <20150714134851.GK13681@linux.intel.com>
Date:	Tue, 14 Jul 2015 09:48:51 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Return the length of a hole from get_block

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.
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