Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH v7 0/8] iov_iter: Improve page extraction (ref, pin or just list) | Date | Mon, 23 Jan 2023 17:19:51 +0000 |
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Matthew Wilcox <willy@infradead.org> 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
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