Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list)

[David Howells]

Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> > 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.

> > 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