On 22/03/2018 16:40, Matthew Wilcox wrote: > On Thu, Mar 22, 2018 at 04:32:00PM +0100, Laurent Dufour wrote: >> On 21/03/2018 23:46, Matthew Wilcox wrote: >>> On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote: >>>> Marking vma as deleted sounds good. The problem for my current approach is >>>> the concurrent page fault may succeed if it access the not yet unmapped >>>> section. Marking deleted vma could tell page fault the vma is not valid >>>> anymore, then return SIGSEGV. >>>> >>>>> does not care; munmap will need to wait for the existing munmap operation >>>> >>>> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right? >>> >>> The other thing about MAP_FIXED that we'll need to handle is unmapping >>> conflicts atomically. Say a program has a 200GB mapping and then >>> mmap(MAP_FIXED) another 200GB region on top of it. So I think page faults >>> are also going to have to wait for deleted vmas (then retry the fault) >>> rather than immediately raising SIGSEGV. >> >> 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. 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. > 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 ?
