Re: copy on write for splice() from file to pipe?

Andy Lutomirski
2023-02-10T15:15:13Z

On Thu, Feb 9, 2023 at 10:58 PM Dave Chinner <david@fromorbit.com> wrote:
>
> On Fri, Feb 10, 2023 at 04:44:41AM +0000, Matthew Wilcox wrote:
> > On Fri, Feb 10, 2023 at 03:06:26PM +1100, Dave Chinner wrote:
> > > So while I was pondering the complexity of this and watching a great
> > > big shiny rocket create lots of heat, light and noise, it occurred
> >
> > That was kind of fun
>
> :)
>
> > > to me that we already have a mechanism for preventing page cache
> > > data from being changed while the folios are under IO:
> > > SB_I_STABLE_WRITES and folio_wait_stable().
> >
> > I thought about bringing that up, but it's not quite right. That works
> > great for writeback, but it only works for writeback. We'd need to track
> > another per-folio counter ... it'd be like the page pinning kerfuffle,
> > only worse.
>
> Hmmm - I didn't think of that. It needs the counter because the
> "stable request" is per folio reference state, not per folio state,
> right? And the single flag works for writeback because we can only
> have one writeback context in progress at a time?
>
> Yeah, not sure how to deal with that easily.
>
> > And for such a rare thing it seems like a poor use of 32
> > bits of per-page state.
>
> Maybe, but zero copy file data -> network send is a pretty common
> workload. Web servers, file servers, remote backup programs, etc.
> Having files being written whilst others are reading them is not as
> common, but that does happen in a wide variety of shared file server
> environments.
>
> Regardless, I just had a couple of ideas - it they don't work so be
> it.
>
> > Not to mention that you can effectively block
> > all writes to a file for an indefinite time by splicing pages to a pipe
> > that you then never read from.
>
> No, I wasn't suggesting that we make pages in transit stable - they
> only need to be stable while the network stack requires them to be
> stable....

This is exactly where the existing splice API is problematic. You
can't splice from a file to a network socket right now. First you
splice to a pipe, and now that pipe contains some magical stuff. And
it stays there potentially forever. Then you splice it again to a
socket.

Would this be better if user code could splice straight to a socket?
At least in principle, there could be a _limited_ amount of time
during which anything needs to wait, and it's fundamentally entirely
reasonable if a concurrent write to a file affects data being
zero-copied to a socket _during the time after the zero-copy send is
requested and before it reports completion_.

Frankly, I really don't like having non-immutable data in a pipe. A
pipe is supposed to be a thing into which bytes are written and out
from which the *same* bytes emerge, at least to the extent that anyone
can observe it. Do we really want:

$ some_program | tee file21 > file2

to potentially write different data to file1 and file2?

--Andy