On 22/03/2018 17:46, Yang Shi wrote: > > > On 3/22/18 9:18 AM, Laurent Dufour wrote: >> >> On 22/03/2018 17:05, Matthew Wilcox wrote: >>> 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. >> Good point, I missed that... >> >>>> 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. >> The PTE locking should prevent that. >> >>> 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. > > We may not need ref_count. After removing "locked-for-deletion" vmas when > mmap(MAP_FIXED) is done, just wake up page fault to re-lookup vma, then it will > find the new vma installed by mmap(MAP_FIXED), right? I do agree, as far as waking up would not require access to the VMA. > I'm not sure if completion can do this or not since I'm not quite familiar with > it :-( I don't know either :/ Laurent. > Yang > >> >>>>> 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. > > > >
