From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Matthew Wilcox <willy@infradead.org>,
Ruan Shiyang <ruansy.fnst@cn.fujitsu.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
"linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"dan.j.williams@intel.com" <dan.j.williams@intel.com>,
"hch@lst.de" <hch@lst.de>, "rgoldwyn@suse.de" <rgoldwyn@suse.de>,
"Qi, Fuli" <qi.fuli@fujitsu.com>,
"Gotou, Yasunori" <y-goto@fujitsu.com>
Subject: Re: 回复: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink
Date: Tue, 28 Apr 2020 08:37:32 -0700 [thread overview]
Message-ID: <20200428153732.GZ6742@magnolia> (raw)
In-Reply-To: <20200428112441.GH2040@dread.disaster.area>
On Tue, Apr 28, 2020 at 09:24:41PM +1000, Dave Chinner wrote:
> On Tue, Apr 28, 2020 at 04:16:36AM -0700, Matthew Wilcox wrote:
> > On Tue, Apr 28, 2020 at 05:32:41PM +0800, Ruan Shiyang wrote:
> > > On 2020/4/28 下午2:43, Dave Chinner wrote:
> > > > On Tue, Apr 28, 2020 at 06:09:47AM +0000, Ruan, Shiyang wrote:
> > > > > 在 2020/4/27 20:28:36, "Matthew Wilcox" <willy@infradead.org> 写道:
> > > > > > On Mon, Apr 27, 2020 at 04:47:42PM +0800, Shiyang Ruan wrote:
> > > > > > > This patchset is a try to resolve the shared 'page cache' problem for
> > > > > > > fsdax.
> > > > > > >
> > > > > > > In order to track multiple mappings and indexes on one page, I
> > > > > > > introduced a dax-rmap rb-tree to manage the relationship. A dax entry
> > > > > > > will be associated more than once if is shared. At the second time we
> > > > > > > associate this entry, we create this rb-tree and store its root in
> > > > > > > page->private(not used in fsdax). Insert (->mapping, ->index) when
> > > > > > > dax_associate_entry() and delete it when dax_disassociate_entry().
> > > > > >
> > > > > > Do we really want to track all of this on a per-page basis? I would
> > > > > > have thought a per-extent basis was more useful. Essentially, create
> > > > > > a new address_space for each shared extent. Per page just seems like
> > > > > > a huge overhead.
> > > > > >
> > > > > Per-extent tracking is a nice idea for me. I haven't thought of it
> > > > > yet...
> > > > >
> > > > > But the extent info is maintained by filesystem. I think we need a way
> > > > > to obtain this info from FS when associating a page. May be a bit
> > > > > complicated. Let me think about it...
> > > >
> > > > That's why I want the -user of this association- to do a filesystem
> > > > callout instead of keeping it's own naive tracking infrastructure.
> > > > The filesystem can do an efficient, on-demand reverse mapping lookup
> > > > from it's own extent tracking infrastructure, and there's zero
> > > > runtime overhead when there are no errors present.
> > > >
> > > > At the moment, this "dax association" is used to "report" a storage
> > > > media error directly to userspace. I say "report" because what it
> > > > does is kill userspace processes dead. The storage media error
> > > > actually needs to be reported to the owner of the storage media,
> > > > which in the case of FS-DAX is the filesytem.
> > >
> > > Understood.
> > >
> > > BTW, this is the usage in memory-failure, so what about rmap? I have not
> > > found how to use this tracking in rmap. Do you have any ideas?
> > >
> > > >
> > > > That way the filesystem can then look up all the owners of that bad
> > > > media range (i.e. the filesystem block it corresponds to) and take
> > > > appropriate action. e.g.
> > >
> > > I tried writing a function to look up all the owners' info of one block in
> > > xfs for memory-failure use. It was dropped in this patchset because I found
> > > out that this lookup function needs 'rmapbt' to be enabled when mkfs. But
> > > by default, rmapbt is disabled. I am not sure if it matters...
> >
> > I'm pretty sure you can't have shared extents on an XFS filesystem if you
> > _don't_ have the rmapbt feature enabled. I mean, that's why it exists.
>
> You're confusing reflink with rmap. :)
>
> rmapbt does all the reverse mapping tracking, reflink just does the
> shared data extent tracking.
>
> But given that anyone who wants to use DAX with reflink is going to
> have to mkfs their filesystem anyway (to turn on reflink) requiring
> that rmapbt is also turned on is not a big deal. Especially as we
> can check it at mount time in the kernel...
Are we going to turn on rmap by default? The last I checked, it did
have a 10-20% performance cost on extreme metadata-heavy workloads.
Or do we only enable it by default if mkfs detects a pmem device?
(Admittedly, most people do not run fsx as a productivity app; the
normal hit is usually 3-5% which might not be such a big deal since you
also get (half of) online fsck. :P)
--D
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com
next prev parent reply other threads:[~2020-04-28 15:40 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-27 8:47 [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 1/8] fs/dax: Introduce dax-rmap btree for reflink Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 2/8] mm: add dax-rmap for memory-failure and rmap Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 3/8] fs/dax: Introduce dax_copy_edges() for COW Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 4/8] fs/dax: copy data before write Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 5/8] fs/dax: replace mmap entry in case of CoW Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 6/8] fs/dax: dedup file range to use a compare function Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 7/8] fs/xfs: handle CoW for fsdax write() path Shiyang Ruan
2020-04-27 8:47 ` [RFC PATCH 8/8] fs/xfs: support dedupe for fsdax Shiyang Ruan
2020-04-27 12:28 ` [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink Matthew Wilcox
2020-04-28 6:09 ` 回复: " Ruan, Shiyang
2020-04-28 6:43 ` Dave Chinner
2020-04-28 9:32 ` Ruan Shiyang
2020-04-28 11:16 ` Matthew Wilcox
2020-04-28 11:24 ` Dave Chinner
2020-04-28 15:37 ` Darrick J. Wong [this message]
2020-04-28 22:02 ` Dave Chinner
2020-06-04 7:37 ` Ruan Shiyang
2020-06-04 14:51 ` Darrick J. Wong
2020-06-05 1:30 ` Dave Chinner
2020-06-05 2:30 ` Ruan Shiyang
2020-06-05 2:11 ` Ruan Shiyang
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