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@xxxxxxxxxxxxx> 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. > 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 > > -- > 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. > > > >
