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Date: Sun, 14 Apr 2024 13:50:16 -0700
From: Luis Chamberlain <mcgrof@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: John Garry <john.g.garry@oracle.com>,
	Pankaj Raghav <p.raghav@samsung.com>,
	Daniel Gomez <da.gomez@samsung.com>,
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Subject: Re: [PATCH v6 00/10] block atomic writes
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Sender: Luis Chamberlain <mcgrof@infradead.org>

On Wed, Apr 10, 2024 at 05:05:20AM +0100, Matthew Wilcox wrote:
> On Mon, Apr 08, 2024 at 10:50:47AM -0700, Luis Chamberlain wrote:
> > On Fri, Apr 05, 2024 at 11:06:00AM +0100, John Garry wrote:
> > > On 04/04/2024 17:48, Matthew Wilcox wrote:
> > > > > > The thing is that there's no requirement for an interface as complex as
> > > > > > the one you're proposing here.  I've talked to a few database people
> > > > > > and all they want is to increase the untorn write boundary from "one
> > > > > > disc block" to one database block, typically 8kB or 16kB.
> > > > > > 
> > > > > > So they would be quite happy with a much simpler interface where they
> > > > > > set the inode block size at inode creation time,
> > > > > We want to support untorn writes for bdev file operations - how can we set
> > > > > the inode block size there? Currently it is based on logical block size.
> > > > ioctl(BLKBSZSET), I guess?  That currently limits to PAGE_SIZE, but I
> > > > think we can remove that limitation with the bs>PS patches.
> > 
> > I can say a bit more on this, as I explored that. Essentially Matthew,
> > yes, I got that to work but it requires a set of different patches. We have
> > what we tried and then based on feedback from Chinner we have a
> > direction on what to try next. The last effort on that front was having the
> > iomap aops for bdev be used and lifting the PAGE_SIZE limit up to the
> > page cache limits. The crux on that front was that we end requiring
> > disabling BUFFER_HEAD and that is pretty limitting, so my old
> > implementation had dynamic aops so to let us use the buffer-head aops
> > only when using filesystems which require it and use iomap aops
> > otherwise. But as Chinner noted we learned through the DAX experience
> > that's not a route we want to again try, so the real solution is to
> > extend iomap bdev aops code with buffer-head compatibility.
> 
> Have you tried just using the buffer_head code?  I think you heard bad
> advice at last LSFMM.  Since then I've landed a bunch of patches which
> remove PAGE_SIZE assumptions throughout the buffer_head code, and while
> I haven't tried it, it might work.  And it might be easier to make work
> than adding more BH hacks to the iomap code.

I have considered it but the issue is that *may work* isn't good enough and
without a test plan for buffer-heads on a real filesystem this may never
suffice. Addressing a buffere-head iomap compat for the block device cache
is less error prone here for now.

  Luis