linux-kernel - Re: 回复: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink

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Message-ID: <259fe633-e1ff-b279-cd8c-1a81eaa40941@cn.fujitsu.com>
Date:   Tue, 28 Apr 2020 17:32:41 +0800
From:   Ruan Shiyang <ruansy.fnst@...fujitsu.com>
To:     Dave Chinner <david@...morbit.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "hch@....de" <hch@....de>, "rgoldwyn@...e.de" <rgoldwyn@...e.de>,
        "Qi, Fuli" <qi.fuli@...itsu.com>,
        "Gotou, Yasunori" <y-goto@...itsu.com>
Subject: Re: 回复: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink



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@...radead.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...

> 
> - if it falls in filesytem metadata, shutdown the filesystem
> - if it falls in user data, call the "kill userspace dead" routines
>    for each mapping/index tuple the filesystem finds for the given
>    LBA address that the media error occurred >
> Right now if the media error is in filesystem metadata, the
> filesystem isn't even told about it. The filesystem can't even shut
> down - the error is just dropped on the floor and it won't be until
> the filesystem next tries to reference that metadata that we notice
> there is an issue.

Understood.  Thanks.

> 
> Cheers,
> 
> Dave.
> 


--
Thanks,
Ruan Shiyang.