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

[Ruan Shiyang <ruansy.fnst@cn.fujitsu.com>]

On 2020/6/4 下午10:51, Darrick J. Wong wrote:
> On Thu, Jun 04, 2020 at 03:37:42PM +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.
>>
>> Hi Dave,
>>
>> I ran into some difficulties when trying to implement the per-extent rmap
>> tracking.  So, I re-read your comments and found that I was misunderstanding
>> what you described here.
>>
>> I think what you mean is: we don't need the in-memory dax-rmap tracking now.
>> Just ask the FS for the owner's information that associate with one page
>> when memory-failure.  So, the per-page (even per-extent) dax-rmap is
>> needless in this case.  Is this right?
> 
> Right.  XFS already has its own rmap tree.
> 
>> Based on this, we only need to store the extent information of a fsdax page
>> in its ->mapping (by searching from FS).  Then obtain the owners of this
>> page (also by searching from FS) when memory-failure or other rmap case
>> occurs.
> 
> I don't even think you need that much.  All you need is the "physical"
> offset of that page within the pmem device (e.g. 'this is the 307th 4k
> page == offset 1257472 since the start of /dev/pmem0') and xfs can look
> up the owner of that range of physical storage and deal with it as
> needed.

Yes, I think so.

> 
>> So, a fsdax page is no longer associated with a specific file, but with a
>> FS(or the pmem device).  I think it's easier to understand and implement.
> 
> Yes.  I also suspect this will be necessary to support reflink...
> 
> --D

OK, Thank you very much.


--
Thanks,
Ruan Shiyang.

> 
>>
>> --
>> Thanks,
>> Ruan Shiyang.
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> - 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.
>>>
>>> Cheers,
>>>
>>> Dave.
>>>
>>
>>
> 
>