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 | Fri, 5 Jun 2020 10:11:51 +0800 |
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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. >>> >> >> > >
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