On Thu 21-05-15 11:09:55, Kent Overstreet wrote: > On Thu, May 21, 2015 at 06:54:53PM +0200, Jan Kara wrote: > > On Wed 20-05-15 18:04:40, Kent Overstreet wrote: > > > > Yeah. I never figured out a sane way to migrate pages and keep everything > > > > else happy. Daniel Phillips is having a go at page forking for tux3; let's > > > > see if the questions about that get resolved. > > > > > > That would be great, we need something. > > > > > > I'd also be really curious what btrfs is doing today - is it just bouncing > > > everything internally, or did they come up with something more clever? > > > > Btrfs is just waiting for IO to complete. > > > > > > > Also, there's probably always going to be situations where we're reading or > > > > > writing to pages user space can stomp on (dio) - IMO we need to add a bio flag > > > > > to annotate this - "if you need this to be stable you have to bounce it". > > > > > Otherwise either filesystems/block drivers are going to be stuck bouncing > > > > > everything, or it'll just (continue to be) buggy. > > > > > > > > Well, for now there's BIO_SNAP_STABLE that forces the block layer to bounce it, > > > > but right now ext3 is the last user of it, and afaict btrfs is the only other > > > > FS that takes care of stable pages on its own. > > > > > > I have no idea what BIO_SNAP_STABLE was supposed to be for, but I don't see how > > > it's useful for anything sane. > > > > It's for the case where lower layer requests it needs stable pages but > > upper layer isn't able to provide them (as is the case of ext3). Then block > > layer bounces the data for the caller. > > > > > But that's the complete opposite of the problem stable pages are supposed to > > > solve: stable pages are for when the _lower_ layer (be it filesystem, bcache, > > > md, lvm) needs the memory being either read to or written from (both, it's not > > > just writes) to not be diddled over while the IO is in flight. > > > > > > Now, a point that I think has been missed is that stable pages are _not_ a > > > complete solution, at least for consumers in the block layer. > > > > > > The situation today is that if I'm in the block layer, and I get a handed a read > > > or write bio, I _don't know_ if it's from something that's going to diddle over > > > those pages or not. So if I require stable pages - be it for data checksumming > > > or for other things - I've just got to bounce the bio myself. > > > > > > And then the really annoying thing is that if you've got stacked things that all > > > need stable pages (maybe btrfs on top of bcache on top of md) - they _all_ have > > > to assume the pages aren't going to be stable, so if they need them they _all_ > > > have to bounce - even though once the first layer bounced the bio that made it > > > stable for everything underneath it. > > > > The current design is that if you need stable pages for your device, set > > bdi capability BDI_CAP_STABLE_WRITES, fs then takes care of not scribbling > > over your page while it is under writeback or uses BIO_SNAP_STABLE if it > > cannot. > > But if I need stable pages, I still have to bounce because that _does not_ > guarantee stable pages, it only gives me stable pages for some of the IOs and in > the lower layers you can't tell which is which. > > Do you see the problem? What good is BDI_CAP_STABLE_WRITES if it's not a > guarantee and I can't tell if I need to bounce or not? So fix the upper layers to make it a guarantee? You mentioned direct IO needs fixing. Anything else? Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
