linux-ext4 - Re: [Bug 207367] Accraid / aptec / Microsemi / ext4 / larger then 16TB

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Date:   Tue, 21 Apr 2020 19:15:53 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        bugzilla-daemon@...zilla.kernel.org, linux-ext4@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [Bug 207367] Accraid / aptec / Microsemi / ext4 / larger then
 16TB

On Tue, Apr 21, 2020 at 09:45:54AM -0700, Christoph Hellwig wrote:
> On Tue, Apr 21, 2020 at 06:29:10PM +0200, Jan Kara wrote:
> > Well, there are two problems with this - firstly, ocfs2 is also using jbd2
> > and it knows nothing about iomap. So that would have to be implemented.
> > Secondly, you have to somehow pass iomap ops to jbd2 so it all boils down
> > to passing some callback to jbd2 during journal init to map blocks anyway
> > as Dave said. And then it is upto filesystem to do the mapping - usually
> > directly using its internal block mapping function - so no need for iomap
> > AFAICT.
> 
> You'll need to describe the mapping some how.  So why not reuse an
> existing mechanism instead of creating a new ad-hoc one?

Well, we could argue that bmap() is an "existing mechanism" --- again,
bmap() returns a u64, so it's perfectly fine.  It's FIBMAP which is
"fundamentally broken", not bmap().  If the goal is to eventually
eliminate bmap() and aops->bmap(), sure, then we should force march
all file systems to use iomap_bmap(), including ocfs2.

Otherwise, if the goal alert users of FIBMAP when it's returning an
corrutped block number, why not move the check if the block is larger
than INT_MAX to ioctl_fibmap() in fs/ioctl.c, instead of in
iomap_bmap()?

If we can't fix this, I'm beginning to think that switching to iomap
for fiemap and bmap is actually a lose for ext4.  It's causing
performance regressions, and now we see it's causing functionality
regressions.  Sure, it's saving a bit of code size, but is it really
worth it to use iomap for fiemap/bmap?

						- Ted