On Wed, 19 Jul 2023 at 19:59, Matt Whitlock [off-list ref] wrote:

On Wednesday, 19 July 2023 06:17:51 EDT, Miklos Szeredi wrote:

quoted

On Thu, 29 Jun 2023 at 17:56, David Howells [off-list ref] wrote:

quoted

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.

Thanks,
Miklos