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From: Goldwyn Rodrigues <rgoldwyn@suse.de>
To: Matthew Wilcox <willy@infradead.org>
Cc: jack@suse.cz, linux-nvdimm@lists.01.org,
	Neal Gompa <ngompa13@gmail.com>,
	darrick.wong@oracle.com, david@fromorbit.com,
	linux-kernel@vger.kernel.org,
	Shiyang Ruan <ruansy.fnst@fujitsu.com>,
	linux-xfs@vger.kernel.org, ocfs2-devel@oss.oracle.com,
	viro@zeniv.linux.org.uk,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	dan.j.williams@intel.com,
	Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: [Ocfs2-devel] [PATCH v2 00/10] fsdax, xfs: Add reflink&dedupe support for fsdax
Date: Wed, 10 Mar 2021 11:04:23 -0600	[thread overview]
Message-ID: <20210310170423.beagx4sowxpa25no@fiona> (raw)
In-Reply-To: <20210310142643.GQ3479805@casper.infradead.org>

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

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  reply	other threads:[~2021-03-10 17:04 UTC|newest]

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-26  0:20 Shiyang Ruan
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 01/10] fsdax: Factor helpers to simplify dax fault code Shiyang Ruan
2021-03-03  9:13   ` Christoph Hellwig
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 02/10] fsdax: Factor helper: dax_fault_actor() Shiyang Ruan
2021-03-03  9:28   ` Christoph Hellwig
2021-03-12  9:01     ` ruansy.fnst
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 03/10] fsdax: Output address in dax_iomap_pfn() and rename it Shiyang Ruan
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 05/10] fsdax: Replace mmap entry in case of CoW Shiyang Ruan
2021-03-03  9:30   ` Christoph Hellwig
2021-03-03  9:41     ` ruansy.fnst
2021-03-03  9:44       ` Christoph Hellwig
2021-03-03  9:48   ` Christoph Hellwig
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 08/10] fsdax: Dedup file range to use a compare function Shiyang Ruan
2021-02-26  8:28   ` Shiyang Ruan
2021-03-03  8:20   ` Joe Perches
2021-03-03  8:45     ` ruansy.fnst
2021-03-03  9:04       ` Joe Perches
2021-03-03  9:39       ` hch
2021-03-03  9:46         ` ruansy.fnst
2021-03-04  5:42   ` [Ocfs2-devel] [RESEND PATCH v2.1 " Shiyang Ruan
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 09/10] fs/xfs: Handle CoW for fsdax write() path Shiyang Ruan
2021-03-03  9:43   ` Christoph Hellwig
2021-03-03  9:57     ` ruansy.fnst
2021-03-03 10:43       ` Christoph Hellwig
2021-03-04  1:35         ` ruansy.fnst
2021-02-26  0:20 ` [Ocfs2-devel] [PATCH v2 10/10] fs/xfs: Add dedupe support for fsdax Shiyang Ruan
2021-02-26  9:45 ` [Ocfs2-devel] Question about the "EXPERIMENTAL" tag for dax in XFS ruansy.fnst
2021-02-26 19:04   ` Darrick J. Wong
2021-02-26 19:24     ` Dan Williams
2021-02-26 20:51       ` Dave Chinner
2021-02-26 20:59         ` Dan Williams
2021-02-26 21:27           ` Dave Chinner
2021-02-26 22:41             ` Dan Williams
2021-02-27 22:36               ` Dave Chinner
2021-02-27 23:40                 ` Dan Williams
2021-02-28 22:38                   ` Dave Chinner
2021-03-01 20:55                     ` Dan Williams
2021-03-01 22:46                       ` Dave Chinner
2021-03-02  0:32                         ` Dan Williams
2021-03-02  2:42                           ` Dave Chinner
2021-03-02  3:33                             ` Dan Williams
2021-03-02  5:38                               ` Dave Chinner
2021-03-02  5:50                                 ` Dan Williams
2021-03-02  3:28                       ` Darrick J. Wong
2021-03-02  5:41                         ` Dan Williams
2021-03-02  7:57                           ` Dave Chinner
2021-03-02 17:49                             ` Dan Williams
2021-03-04 23:40                               ` Darrick J. Wong
2021-03-01  7:26       ` Yasunori Goto
2021-03-01 21:34         ` Dan Williams
     [not found] ` <20210226002030.653855-5-ruansy.fnst@fujitsu.com>
2021-03-03  9:29   ` [Ocfs2-devel] [PATCH v2 04/10] fsdax: Introduce dax_iomap_cow_copy() Christoph Hellwig
     [not found] ` <20210226002030.653855-7-ruansy.fnst@fujitsu.com>
2021-03-03  9:31   ` [Ocfs2-devel] [PATCH v2 06/10] fsdax: Add dax_iomap_cow_copy() for dax_iomap_zero Christoph Hellwig
     [not found] ` <20210226002030.653855-8-ruansy.fnst@fujitsu.com>
2021-02-26  4:14   ` [Ocfs2-devel] [PATCH v2 07/10] iomap: Introduce iomap_apply2() for operations on two files Darrick J. Wong
2021-02-26  8:11     ` ruansy.fnst
2021-02-26  8:25   ` Shiyang Ruan
2021-03-04  5:41   ` [Ocfs2-devel] [RESEND PATCH v2.1 " Shiyang Ruan
2021-03-11 12:30     ` Christoph Hellwig
2021-03-09  6:36 ` [Ocfs2-devel] [PATCH v2 00/10] fsdax, xfs: Add reflink&dedupe support for fsdax Xiaoguang Wang
2021-03-10  1:32   ` ruansy.fnst
2021-03-09 16:19 ` Goldwyn Rodrigues
2021-03-10  1:26   ` ruansy.fnst
2021-03-10 12:30 ` Neal Gompa
2021-03-10 13:02   ` Matthew Wilcox
2021-03-10 13:36     ` Neal Gompa
2021-03-10 13:55       ` Matthew Wilcox
2021-03-10 14:21     ` Goldwyn Rodrigues
2021-03-10 14:26       ` Matthew Wilcox
2021-03-10 17:04         ` Goldwyn Rodrigues [this message]
2021-03-11  0:53         ` Dan Williams
2021-03-11  8:26           ` Neal Gompa
2021-03-13 13:07         ` Adam Borowski
2021-03-13 16:24           ` Neal Gompa
2021-03-13 22:00             ` Adam Borowski

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