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 10:30:57 +0800 |
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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. >
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