Matthew Wilcox [off-list ref] wrote:

quoted

Wouldn't that potentially make someone's entire malloc() heap entirely NOCOW
if they did a single DIO to/from it.

Yes.  Would that be an actual problem for any real application?

Without auditing all applications that do direct I/O writes, it's hard to
say - but a big database engine, Oracle for example, forking off a process,
say, could cause a massive slow down as fork suddenly has to copy a huge
amount of malloc'd data unnecessarily[*].

[*] I'm making wild assumptions about how Oracle's DB engine works.

quoted

Also you only mention DIO read - but what about "start DIO write; fork();
touch buffer" in the parent - now the write buffer belongs to the child
and they can affect the parent's write.

I'm struggling to see the problem here.  If the child hasn't exec'd, the
parent and child are still in the same security domain.  The parent
could have modified the buffer before calling fork().

It could still inadvertently change the data its parent set to write out.  The
child *shouldn't* be able to change the parent's in-progress write.  The most
obvious problem would be in something that does DIO from a stack buffer, I
think.

David