Messages in this thread | | | Subject | Re: 回复: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink | From | Ruan Shiyang <> | Date | Thu, 4 Jun 2020 15:37:42 +0800 |
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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?
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.
-- 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. >
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