On Thu, Mar 22, 2018 at 04:54:52PM +0100, Laurent Dufour wrote: > On 22/03/2018 16:40, Matthew Wilcox wrote: > > On Thu, Mar 22, 2018 at 04:32:00PM +0100, Laurent Dufour wrote: > >> Regarding the page fault, why not relying on the PTE locking ? > >> > >> When munmap() will unset the PTE it will have to held the PTE lock, so this > >> will serialize the access. > >> If the page fault occurs before the mmap(MAP_FIXED), the page mapped will be > >> removed when mmap(MAP_FIXED) would do the cleanup. Fair enough. > > > > The page fault handler will walk the VMA tree to find the correct > > VMA and then find that the VMA is marked as deleted. If it assumes > > that the VMA has been deleted because of munmap(), then it can raise > > SIGSEGV immediately. But if the VMA is marked as deleted because of > > mmap(MAP_FIXED), it must wait until the new VMA is in place. > > I'm wondering if such a complexity is required. > If the user space process try to access the page being overwritten through > mmap(MAP_FIXED) by another thread, there is no guarantee that it will > manipulate the *old* page or *new* one. Right; but it must return one or the other, it can't segfault. > I'd think this is up to the user process to handle that concurrency. > What needs to be guaranteed is that once mmap(MAP_FIXED) returns the old page > are no more there, which is done through the mmap_sem and PTE locking. Yes, and allowing the fault handler to return the *old* page risks the old page being reinserted into the page tables after the unmapping task has done its work. 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 think I was wrong to describe VMAs as being *deleted*. I think we > > instead need the concept of a *locked* VMA that page faults will block on. > > Conceptually, it's a per-VMA rwsem, but I'd use a completion instead of > > an rwsem since the only reason to write-lock the VMA is because it is > > being deleted. > > Such a lock would only makes sense in the case of mmap(MAP_FIXED) since when > the VMA is removed there is no need to wait. Isn't it ? I can't think of another reason. I suppose we could mark the VMA as locked-for-deletion or locked-for-replacement and have the SIGSEGV happen early. But I'm not sure that optimising for SIGSEGVs is a worthwhile use of our time. Just always have the pagefault sleep for a deleted VMA.
