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
