On Tue, Apr 28, 2020 at 09:24:41PM +1000, Dave Chinner wrote: > On Tue, Apr 28, 2020 at 04:16:36AM -0700, Matthew Wilcox wrote: > > On Tue, Apr 28, 2020 at 05:32:41PM +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. > > > > > > > > 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. > > > > > > Understood. > > > > > > BTW, this is the usage in memory-failure, so what about rmap? I have not > > > found how to use this tracking in rmap. Do you have any ideas? > > > > > > > > > > > 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. > > > > > > I tried writing a function to look up all the owners' info of one block in > > > xfs for memory-failure use. It was dropped in this patchset because I found > > > out that this lookup function needs 'rmapbt' to be enabled when mkfs. But > > > by default, rmapbt is disabled. I am not sure if it matters... > > > > I'm pretty sure you can't have shared extents on an XFS filesystem if you > > _don't_ have the rmapbt feature enabled. I mean, that's why it exists. > > You're confusing reflink with rmap. :) > > rmapbt does all the reverse mapping tracking, reflink just does the > shared data extent tracking. > > But given that anyone who wants to use DAX with reflink is going to > have to mkfs their filesystem anyway (to turn on reflink) requiring > that rmapbt is also turned on is not a big deal. Especially as we > can check it at mount time in the kernel... Are we going to turn on rmap by default? The last I checked, it did have a 10-20% performance cost on extreme metadata-heavy workloads. Or do we only enable it by default if mkfs detects a pmem device? (Admittedly, most people do not run fsx as a productivity app; the normal hit is usually 3-5% which might not be such a big deal since you also get (half of) online fsck. :P) --D > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
