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Date:   Wed, 10 Mar 2021 11:04:23 -0600
From:   Goldwyn Rodrigues <rgoldwyn@suse.de>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Neal Gompa <ngompa13@gmail.com>,
        Shiyang Ruan <ruansy.fnst@fujitsu.com>,
        linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-nvdimm@lists.01.org,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        darrick.wong@oracle.com, dan.j.williams@intel.com, jack@suse.cz,
        viro@zeniv.linux.org.uk, Btrfs BTRFS <linux-btrfs@vger.kernel.org>,
        ocfs2-devel@oss.oracle.com, david@fromorbit.com, hch@lst.de
Subject: Re: [PATCH v2 00/10] fsdax,xfs: Add reflink&dedupe support for fsdax
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On 14:26 10/03, Matthew Wilcox wrote:
> On Wed, Mar 10, 2021 at 08:21:59AM -0600, Goldwyn Rodrigues wrote:
> > On 13:02 10/03, Matthew Wilcox wrote:
> > > On Wed, Mar 10, 2021 at 07:30:41AM -0500, Neal Gompa wrote:
> > > > Forgive my ignorance, but is there a reason why this isn't wired up to
> > > > Btrfs at the same time? It seems weird to me that adding a feature
> > > 
> > > btrfs doesn't support DAX.  only ext2, ext4, XFS and FUSE have DAX support.
> > > 
> > > If you think about it, btrfs and DAX are diametrically opposite things.
> > > DAX is about giving raw access to the hardware.  btrfs is about offering
> > > extra value (RAID, checksums, ...), none of which can be done if the
> > > filesystem isn't in the read/write path.
> > > 
> > > That's why there's no DAX support in btrfs.  If you want DAX, you have
> > > to give up all the features you like in btrfs.  So you may as well use
> > > a different filesystem.
> > 
> > DAX on btrfs has been attempted[1]. Of course, we could not
> 
> But why?  A completeness fetish?  I don't understand why you decided
> to do this work.

If only I had a penny every time I heard "why would you want to do that?"

> 
> > have checksums or multi-device with it. However, got stuck on
> > associating a shared extent on the same page mapping: basically the
> > TODO above dax_associate_entry().
> > 
> > Shiyang has proposed a way to disassociate existing mapping, but I
> > don't think that is the best solution. DAX for CoW will not work until
> > we have a way of mapping a page to multiple inodes (page->mapping),
> > which will convert a 1-N inode-page mapping to M-N inode-page mapping.
> 
> If you're still thinking in terms of pages, you're doing DAX wrong.
> DAX should work without a struct page.

Not pages specifically, but mappings.
fsdax needs the mappings during the page fault and it breaks in case both
files fault on the same shared extent.

For Reference: WARN_ON_ONCE(page->mapping && page->mapping != mapping)
in dax_disassociate_entry().

-- 
Goldwyn