On Thu, Mar 22, 2018 at 05:18:55PM +0100, Laurent Dufour wrote: > > It's *really* rare to page-fault on a VMA which is in the middle of > > being replaced. Why are you trying to optimise it? > > I was not trying to optimize it, but to not wait in the page fault handler. > This could become tricky in the case the VMA is removed once mmap(MAP_FIXED) is > done and before the waiting page fault got woken up. This means that the > removed VMA structure will have to remain until all the waiters are woken up > which implies ref_count or similar. Yes, that's why we don't want an actual rwsem. What I had in mind was a struct completion on the stack of the caller of munmap(), and a pointer to it from the vma. The page fault handler grabs the VMA tree lock, walks the VMA tree and finds a VMA. If the VMA is marked as locked, it waits for the completion. Upon wakeup *it does not look at the VMA*, instead it restarts the page fault.
