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SubjectRe: 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" <> 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?

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.

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.

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.

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