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 2020/6/5 上午9:30, Dave Chinner wrote:
> On Thu, Jun 04, 2020 at 07:51:07AM -0700, 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.
> 
> *nod*
> 
>>> 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.
> 
> Right. If we have the dax device associated with the page that had
> the failure, then we can determine the offset of the page into the
> block device address space and that's all we need to find the owner
> of the page in the filesystem.
> 
> Note that there may actually be no owner - the page that had the
> fault might land in free space, in which case we can simply zero
> the page and clear the error.

OK.  Thanks for pointing out.

> 
>>> 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.
> 
> Effectively, yes. But we shouldn't need to actually associate the
> page with anything at the filesystem level because it is already
> associated with a DAX device at a lower level via a dev_pagemap.
> The hardware page fault already runs thought this code
> memory_failure_dev_pagemap() before it gets to the DAX code, so
> really all we need to is have that function pass us the page, offset
> into the device and, say, the struct dax_device associated with that
> page so we can get to the filesystem superblock we can then use for
> rmap lookups on...
> 

OK.  I was just thinking about how can I execute the FS rmap search from 
the memory-failure.  Thanks again for pointing out. :)


--
Thanks,
Ruan Shiyang.

> Cheers,
> 
> Dave.
>