Adding Jens, because he's one of the main splice people. You do seem to be stepping on his work ;) Jens, see https://lore.kernel.org/lkml/0cfd9f02-dea7-90e2-e932-c8129b6013c7@xxxxxxxxx On Thu, Feb 9, 2023 at 5:56 AM Stefan Metzmacher <metze@xxxxxxxxx> wrote: > > So we have two cases: > > 1. network -> socket -> splice -> pipe -> splice -> file -> storage > > 2. storage -> file -> splice -> pipe -> splice -> socket -> network > > With 1. I guess everything can work reliable [..] > > But with 2. there's a problem, as the pages from the file, > which are spliced into the pipe are still shared without > copy on write with the file(system). Well, honestly, that's really the whole point of splice. It was designed to be a way to share the storage data without having to go through a copy. > I'm wondering if there's a possible way out of this, maybe triggered by a new > flag passed to splice. Not really. So basically, you cannot do "copy on write" on a page cache page, because that breaks sharing. You *want* the sharing to break, but that's because you're violating what splice() was for, but think about all the cases where somebody is just using mmap() and expects to see the file changes. You also aren't thinking of the case where the page is already mapped writably, and user processes may be changing the data at any time. > I looked through the code and noticed the existence of IOMAP_F_SHARED. Yeah, no. That's a hacky filesystem thing. It's not even a flag in anything core like 'struct page', it's just entirely internal to the filesystem itself. > Is there any other way we could archive something like this? I suspect you simply want to copy it at splice time, rather than push the page itself into the pipe as we do in copy_page_to_iter_pipe(). Because the whole point of zero-copy really is that zero copy. And the whole point of splice() was to *not* complicate the rest of the system over-much, while allowing special cases. Linux is not the heap of bad ideas that is Hurd that does various versioning etc, and that made copy-on-write a first-class citizen because it uses the concept of "immutable mapped data" for reads and writes. Now, I do see a couple of possible alternatives to "just create a stable copy". For example, we very much have the notion of "confirm buffer data before copying". It's used for things like "I started the IO on the page, but the IO failed with an error, so even though I gave you a splice buffer, it turns out you can't use it". And I do wonder if we could introduce a notion of "optimistic splice", where the splice works exactly the way it does now (you get a page reference), but the "confirm" phase could check whether something has changed in that mapping (using the file versioning or whatever - I'm hand-waving) and simply fail the confirm. That would mean that the "splice to socket" part would fail in your chain, and you'd have to re-try it. But then the onus would be on *you* as a splicer, not on the rest of the system to fix up your special case. That idea sounds fairly far out there, and complicated and maybe not usable. So I'm just throwing it out as a "let's try to think of alternative solutions". Anybody? Linus
