lkml.org 
[lkml]   [2020]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: 回复: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink
From
Date


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.
>>>
>>
>>
>
>


\
 
 \ /
  Last update: 2020-06-05 04:13    [W:0.086 / U:2.500 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site