Re: [RFC PATCH 1/4] splice: Fix corruption of spliced data after splice() returns

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 19 Jul 2023 at 21:44, Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Jul 19, 2023 at 09:35:33PM +0200, Miklos Szeredi wrote:
> > On Wed, 19 Jul 2023 at 19:59, Matt Whitlock <kernel@mattwhitlock.name> wrote:
> > >
> > > On Wednesday, 19 July 2023 06:17:51 EDT, Miklos Szeredi wrote:
> > > > On Thu, 29 Jun 2023 at 17:56, David Howells <dhowells@redhat.com> wrote:
> > > >>
> > > >> Splicing data from, say, a file into a pipe currently leaves the source
> > > >> pages in the pipe after splice() returns - but this means that those pages
> > > >> can be subsequently modified by shared-writable mmap(), write(),
> > > >> fallocate(), etc. before they're consumed.
> > > >
> > > > What is this trying to fix?   The above behavior is well known, so
> > > > it's not likely to be a problem.
> > >
> > > Respectfully, it's not well-known, as it's not documented. If the splice(2)
> > > man page had mentioned that pages can be mutated after they're already
> > > ostensibly at rest in the output pipe buffer, then my nightly backups
> > > wouldn't have been incurring corruption silently for many months.
> >
> > splice(2):
> >
> >        Though we talk of copying, actual copies are generally avoided.
> > The kernel does this by implementing a pipe buffer as a set  of
> > refer‐
> >        ence-counted  pointers  to  pages  of kernel memory.  The
> > kernel creates "copies" of pages in a buffer by creating new pointers
> > (for the
> >        output buffer) referring to the pages, and increasing the
> > reference counts for the pages: only pointers are copied, not the
> > pages of the
> >        buffer.
> >
> > While not explicitly stating that the contents of the pages can change
> > after being spliced, this can easily be inferred from the above
> > semantics.
>
> So what's the API that provides the semantics of _copying_?

What's your definition of copying?

Thanks,
Miklos